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Meaning of Celestial Art: Celestial artwork builds on the themes inherent in the structure of the universe: galaxies, nebulae, planets, suns and solar systems. These are some of the oldest themes known to man, though we are seeing them through the Hubble telescope as though it was for the first time. Understanding our universe inspires awe and wonder. Creating celestial artwork is a meditation on the immensity and the beauty of the universe itself.

Meaning of the Sun and Moon: The sun is the most miraculous of all objects in the sky. The sun in art is a theme that lifts our spirits and gives us hope. The sun is an endlessly fascinating subject. New photos being returned by satellites and telescopes give us ever new views of the sun and increasing amounts of materials for the creative artist to depict the sun in art. The moon has also been the focus of song and superstition for millennia as the ruling power and light of the night sky.

CELESTIAL ARTWORK

Celestial Artwork Depicting Galaxies	Getting Images from Deep Space	Science of the Hubble Telescope
Celestial Artwork and the Color of Stars	Designing to Fly in Space	Seeing Infinity
The Birth and Growth of Galaxies	The Creation of Color in Hubble Images	How Stars Get Their Names
More About Galaxies	Essentials of the Hubble Story	Best Art in the Universe
The Hubble Team	Mysteries of the Universe

Celestial Artwork Depicting Galaxies
Galaxies are the largest formation in the universe, composed of hundreds of billions of stars. A galaxy also contains patterns of gases and dust, black holes and many frequencies of light. A galaxy may be as large as 200,000 light years across. The recognizable structures of familiar galaxies provide nearly endless visual material for celestial artwork.

Our own galaxy is the Milky Way. All of the visible stars (unaided by a telescope) are part of the Milky Way, which is more dense in some areas and less dense in others, giving it spiral arms. Earth and our entire solar system exists at the edge of a spiral arm called the Orion Arm. At the center of the arms is a disk or bulge. The closest spiral galaxy to ours is the Andromeda galaxy. The spiral formation is a fascinating image and theme for celestial art. Scientists believe that the Milky Way and the Andromeda Gallaxy will collide five billion years in the future. Elliptical galaxies have a central region and a halo effect, but they do not have a disk.

The following description of the Milky Way is taken from HubbleSite.org:

The disk is a flattened region that surrounds the bulge in a spiral galaxy. The disk is shaped like a pancake. The Milky Way's disk is 100,000 light years across and 1,000 light years thick. It contains mostly young stars, gas and dust, which are concentrated in spiral arms. Some old stars are also present.

The spiral arms are curved extensions that begin at the bulge of a spiral galaxy, giving it a "pinwheel" appearance. Spiral arms contain a lot of gas and dust as well as young blue stars. Spiral arms are found only in spiral galaxies.

The halo primarily contains individual old stars and clusters of old stars ("globular clusters"). The halo also contains "dark matter," which is material that we cannot see but whose gravitational force can be measured. The Milky Way's halo may be over 130,000 light years across.*

Courtesy of HubbleSite.org. STScI is operated by the Association of Universities for research in Astronomy, Inc. under a contract with the National Aeronautics and Space Administration. Material credited to STScI was created, authored, and/or prepared for NASA under Contract NAS5-26555. Unless otherwise specifically stated, no claim to copyright is being asserted by STScI and it may be freely used as in the public domain in accordance with NASA's contract.

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Celestial Artwork and the Colors of Stars
Celestial artwork takes advantage of the patterns of stars as well as the patterns of gases. Some stars appear to be scattered across the canvas while others are in recognizable formations. This creates a contrast in celestial artwork between what appears to be planned images and accidental or unplanned images. The fluidity of celestial artwork gives the sense of spaciousness and structure simultaneously.

Stars come in a variety of colors including blue and red. Blue stars are very hot and therefor tend to have a shorter lifespan. Red stars are cooler and last longer. Regions of galaxies that have experienced recent star formations tend to be bluer than older star regions. The colors of stars and the surrounding gases lends color to celestial artwork.

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The Birth and Growth of Galaxies*

NASA's Hubble Space Telescope is uncovering important new clues to a galaxy's birth and growth by peering into its heart — a bulge of millions of stars that resembles a bulbous center yolk in the middle of a disk of egg white.

Hubble astronomers are trying to solve the mystery of which came first: the stellar disk or the central bulge?

Two complementary surveys by independent teams of astronomers using Hubble show that the hubs of some galaxies formed early in the Universe, while others formed more slowly, across a long stretch of time.

Hubble confirms that the evolutionary paths of bulges and disks are connected. The central bulge stabilizes a galaxy's development and largely controls the ebb and flow of star birth in the core. The central bulge holds secrets as to how and when a galaxy formed. Before Hubble, astronomers had detailed information only about the complex core of our galaxy, which has a small bulge peppered with massive young star clusters and a telltale bar structure funneling gas to the center. Hubble allows astronomers to see bright star clusters, bars and other structures deep inside the bulges of other galaxies.

A group led by Reynier Peletier from the University of Nottingham, in the United Kingdom, has confirmed that the central bulges of more tightly wound spirals were all created at more or less the same time in the early universe.

A second team, led by C. Marcella Carollo of Columbia University in New York, surveyed galaxies that have small bulges and bar-like structures that bisect the nucleus like the slash across a no-smoking sign. They found that the bulges in these galaxies grew more recently, through markedly different processes happening within the galaxy's disk.

Both surveys used Hubble's precise resolution to peer into bulbous hubs of more than 200 neighboring galaxies, out to a distance of 100 million light-years. Using Hubble's visible-light and infrared cameras to penetrate deep into the cores of the galaxies, astronomers were able to untangle the stars' true colors — a measure of age — from their apparent colors, which are made redder by interstellar dust.

Peletier's team used Hubble to look into the center of 20 spiral galaxies that have large bulges. The team found that elliptical bulges of stars formed over a relatively brief period very early in the young universe. This could have happened through the collapse of a single cloud of hydrogen or merger of primeval star clusters.

"Apparently everywhere in the universe these intermediate-sized galaxies must have started forming early on," reports Peletier in a paper to be published in the Monthly Notices of the Royal Astronomical Society. "The bulges of early spiral galaxies are old, and at least the outer parts of their disks are considerably younger."

Carollo's team found that in a different class of spiral galaxy, a small bulge probably formed early on, but was later fed by gas flowing into the galaxy's core, likely along a bar-like structure caused by instabilities in the surrounding disk of stars. The gas fueled the birth of new stars, and the bulge inflated like a beach ball as brilliant star clusters populated the center.

Carollo's results, to be published in the Astrophysical Journal, show young and old stars in the bulge. The researchers say that these types of bulges can continue to grow in galaxies in the present universe, but it is unlikely that they will ever become as big as those giant bulges that formed when the universe was young.

The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

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More About Galaxies*

Astronomer Edwin Hubble classified galaxies into four major types: spiral, barred spiral, elliptical and irregular. Most of the nearby, bright galaxies are spirals, barred spirals or ellipticals.

Spiral galaxies have a bulge at the center and a flattened disk containing spiral arms. Spiral galaxies have a variety of shapes and are classified according to the size of the bulge and the tightness and appearance of the arms. The spiral arms, which wrap around the bulge, contain numerous young blue stars and lots of gas and dust. Stars in the bulge tend to be older and redder. Yellow stars like our Sun are found throughout the disk of a spiral galaxy. The disks of spiral galaxies rotate somewhat like a hurricane or a whirlpool.

Barred spiral galaxies are spiral galaxies that have a bar-shaped collection of stars running across the center of the galaxy.

Elliptical galaxies do not have a disk or arms. Instead, they are characterized by a smooth, oval-shaped appearance. Ellipticals contain old stars, and possess little gas or dust. They are classified by the shape of the ball, which can range from round to oval (baseball-shaped to football-shaped). In contrast to the disks of spirals, the stars in ellipticals do not all revolve around the center in an organized way. The stars move on randomly oriented orbits within the galaxy, like a swarm of bees.

Irregular galaxies are galaxies that are neither spiral nor elliptical. They tend to be smaller objects that are without definite shape, and tend to have very hot newer stars mixed in with lots of gas and dust.

The Hubble Team*

All of the Hubble Space Telescope's activities are controlled by people on the ground. The focal point of all Hubble operations is the Flight Operations Team (FOT), which is located at the Goddard Space Flight Center in Greenbelt, Maryland. Here, Hubble's controllers monitor the telescope's health while overseeing its movements and science activities. The controllers direct Hubble's movements by sending commands via satellite to the telescope's onboard computer. The majority of Hubble's operations are programmed in advance, but controllers can also interact in real time with the spacecraft, telling it what to do and when to do it.

Hubble's flight operations facility operates 24 hours a day, 7 days a week. The specially trained engineers and technicians who comprise the FOT work rotating shifts, with 3 to 4 people on each shift. A typical day involves commanding and pointing the telescope, monitoring its behavior on consoles, and looking for anything unusual in the technical sense.

The ground controllers become even busier than usual during Hubble servicing missions. Shortly after the shuttle is launched, the controllers instruct Hubble to stop normal science operations. To prepare the huge telescope for rendezvous and capture, they command Hubble's aperture door to close and its high gain antennas to be stowed. After capture, as the astronauts install new equipment on Hubble, the controllers immediately test the updates. Later, while the crew sleeps, controllers perform more detailed reviews. At the end of each servicing mission, the Flight Operations Team deploy Hubble's high-gain antennas and open its aperture door. They then reactivate all Hubble equipment powered off during the servicing call.

After that, the telescope undergoes a check-out period, called the Servicing Mission Orbital Verification (SMOV), during which all new instruments are put through their paces to ensure that everything is operating as expected.

Each year astronomers from dozens of countries vie for precious minutes of Hubble's unrivaled view of the cosmos. Astronomers from around the world submit observing proposals to the Space Telescope Science Institute (STScI).

A review committee made up of experts from the astronomical community determines which proposed observations address pressing scientific questions and make the best use of the telescope's capabilities. Each year more than 1,000 proposals are reviewed and approximately 200 are selected, which represents roughly 20,000 individual observations.

Scheduling of the viewing time falls to staff at STScI. Once a proposal is accepted, the observation is carefully planned — along with thousands of others — for the most appropriate viewing time. Planning viewing time is tricky because there are certain times of the year during which Earth's revolution around the Sun causes a "geometry" in which the target is too close to the Sun to be observable.

Consequently, technicians must schedule each observation down to a fraction of a second. Observation information such as which instrument to use, what filter to use, and how long the exposure should be must be converted into a detailed technical list of second-by-second instructions. These instructions are loaded onto the telescope's computers a few days before the scheduled observation.

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Getting Images from Deep Space*

A quartet of antennae on the telescope sends and receives information between Hubble and the Flight Operations Team at the Goddard Space Flight Center in Greenbelt, Md. Engineers use satellites to communicate with the telescope, giving it directions and commands. The telescope has two main computers and a number of smaller systems. One of the main computers handles the commands that point the telescope and other system-wide functions. The other talks to the instruments, receives their data, and sends it to satellites that in turn transmit it to the ground.

Once the ground station transfers the data to Goddard, Goddard sends it to the Space Telescope Science Institute (STScI), where staff translate the data into scientifically meaningful units — such as wavelength or brightness — and archive the information on 5.25-inch magneto-optical disks. Hubble sends the archive enough information to fill about 18 DVDs every week. Astronomers can download archived data via the Internet and analyze it from anywhere in the world.

Hundreds of engineers and computer scientists at Goddard Space Flight Center and STScI are responsible for keeping Hubble operating and monitoring its safety, health and performance. At Goddard, controllers monitor the telescope's health while they direct its movements and science activities. STScI staff also schedule use of the telescope, monitor and calibrate the instruments, operate the archive and conduct public outreach.

Astronomers from around the world compete for time to use Hubble. More scientists want to use the telescope than there is time to use it, so a review committee of astronomy experts has to pick out the best proposals from the bunch. The winning proposals are the ones that make the best use of the telescope’s capabilities while addressing pressing astronomical questions. Each year around 1,000 proposals are reviewed and approximately 200 are selected, for a total of 20,000 individual observations.

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Designing to Fly in Space: The Hubble Space Telescope*

Designers of the Hubble Space Telescope had to take into account the conditions in which it was to operate. Hubble would be subject to the rigors of zero gravity and temperature extremes — fluctuations of more than 100 degrees Fahrenheit during each trip around Earth.

To accommodate this less-than-hospitable operating environment, Hubble was given a "skin," or blanket, of multilayered insulation (MLI), which protects the telescope from temperature extremes. During Servicing Mission 4 in 2009, astronauts also added panels of insulation called New Outer Blanket Layers (NOBLs) over portions of Hubble. NOBLs replaced sections of blanket that had broken down from exposure to the harsh conditions of space. Beneath Hubble's insulation is a lightweight aluminum shell, which provides an external structure to the spacecraft and houses its optical system and science instruments.

Hubble's optical system is held together by a truss (supporting "skeleton") measuring 210 in (5.3 m) in length and 115 in (2.9 m) in diameter. The 252 lb (114 kg) truss is made of graphite epoxy — the same material used in many golf clubs, tennis racquets and bicycles. Graphite epoxy is a stiff, strong, and lightweight material that resists expanding and contracting in extremes of temperature.

In order to run all the many subsystems onboard the Hubble, several computers and microprocessors reside in the Hubble body and in each science instrument. Two main computers, which girdle Hubble's "waist," direct the show. One talks to the instruments, receives their data and telemetry, sends the data off to interface units for transmission to the ground, and sends commands and timing information to the instruments. The other main computer handles the gyroscopes, the pointing control subsystem, and other system-wide functions. Special backup computers keep Hubble safe in the event of a problem.

Each instrument itself also houses small computers and microprocessors, which direct their activities. These computers direct the rotation of filter wheels, open and close exposure shutters, maintain the temperature of the instruments, collect data and talk to the main computers.

Hubble needs electricity to operate. Since it can't be plugged in to a ground-based power source, it runs on sunlight, making it the ultimate cordless power tool. Flanking the telescope's tube are two thin, blue solar arrays. Each wing-like array has a solar cell "blanket" that converts the Sun's energy into 2,800 watts of electricity. The solar arrays convert sunlight directly into electricity to run the telescope's scientific instruments, computers and radio transmitters.

Some of the energy generated is stored in onboard batteries so the telescope can operate while it's in Earth's shadow (which is about 36 minutes out of each 97-minute orbit). Fully charged, each battery contains enough energy to sustain the telescope in normal science operations mode for 7.5 hours, or five orbits.

The solar arrays are designed for replacement by visiting astronauts. They can be folded for shuttle trips to and from Hubble.

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The Creation of Color in Hubble Images

Taking color pictures with the Hubble Space Telescope is much more complex than taking color pictures with a traditional camera. For one thing, Hubble doesn't use color film — in fact, it doesn't use film at all. Rather, its cameras record light from the universe with special electronic detectors. These detectors produce images of the cosmos not in color, but in shades of black and white.

Finished color images are actually combinations of two or more black-and-white exposures to which color has been added during image processing.

The colors in Hubble images, which are assigned for various reasons, aren't always what we'd see if we were able to visit the imaged objects in a spacecraft. We often use color as a tool, whether it is to enhance an object's detail or to visualize what ordinarily could never be seen by the human eye.

Color in Hubble images is used to highlight interesting features of the celestial object being studied. It is added to the separate black-and-white exposures that are combined to make the final image.

Creating color images out of the original black-and-white exposures is equal parts art and science.

We use color:

Light from astronomical objects comes in a wide range of colors, each corresponding to a particular kind of electromagnetic wave. Hubble can detect all the visible wavelengths of light plus many more that are invisible to human eyes, such as ultraviolet and infrared light.

Astronomical objects often look different in these different wavelengths of light. To record what an object looks like at a certain wavelength, Hubble uses special filters that allow only a certain range of light wavelengths through. Once the unwanted light has been filtered out, the remaining light is recorded.

Hubble's many filters allow it to record images in a variety of wavelengths of light. Since the cameras can detect light outside the visible light spectrum, the use of filters allows scientists to study "invisible" features of objects — those only visible in ultraviolet and infrared wavelengths.

... Hubble isolates these specific wavelengths using special filters. Choosing a particular filter reveals an image of the galaxy taken through that filter — that is, in a specific wavelength range. The finished image...is actually a combination of all the filtered images.

Many full-color Hubble images are combinations of three separate exposures — one each taken in red, green, and blue light. When mixed together, these three colors of light can simulate almost any color of light that is visible to human eyes. That’s how televisions, computer monitors, and video cameras recreate colors.

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History's Most Important Observatory: Essentials of the Hubble Story*

Since the earliest days of astronomy, since the time of Galileo, astronomers have shared a single goal — to see more, see farther, see deeper.

The Hubble Space Telescope's launch in 1990 sped humanity to one of its greatest advances in that journey. Hubble is a telescope that orbits Earth. Its position above the atmosphere, which distorts and blocks the light that reaches our planet, gives it a view of the universe that typically far surpasses that of ground-based telescopes.

Hubble is one of NASA's most successful and long-lasting science missions. It has beamed hundreds of thousands of images back to Earth, shedding light on many of the great mysteries of astronomy. Its gaze has helped determine the age of the universe, the identity of quasars, and the existence of dark energy.

Hubble's discoveries have transformed the way scientists look at the universe. Its ability to show the universe in unprecedented detail has turned astronomical conjectures into concrete certainties. It has winnowed down the collection of theories about the universe even as it sparked new ones, clarifying the path for future astronomers.

Among its many discoveries, Hubble has revealed the age of the universe to be about 13 to 14 billion years, much more accurate than the old range of anywhere from 10 to 20 billion years. Hubble played a key role in the discovery of dark energy, a mysterious force that causes the expansion of the universe to accelerate.

Hubble has shown scientists galaxies in all stages of evolution, including toddler galaxies that were around when the universe was still young, helping them understand how galaxies form. It found protoplanetary disks, clumps of gas and dust around young stars that likely function as birthing grounds for new planets. It discovered that gamma-ray bursts — strange, incredibly powerful explosions of energy — occur in far-distant galaxies when massive stars collapse. And these are only a handful of its many contributions to astronomy.

The sheer amount of astronomy based on Hubble observations has also helped make it one of history's most important observatories. More than 6,000 scientific articles have been published based on Hubble data.

The policies that govern the telescope have contributed to its incredible productivity. The telescope is an instrument for the entire astronomical community — any astronomer in the world can submit a proposal and request time on the telescope. Teams of experts then select the observations to be performed. Once observations are completed, the astronomers have a year to pursue their work before the data is released to the entire scientific community. Because everyone gets to see the information, the observations have given rise to a multitude of findings — many in areas that would not have been predicted by the telescope’s original proposals. Hubble's success with these policies has helped spread them throughout the astronomical community, and they are becoming common with other observatories.

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Science of the Hubble Telescope*

The Hubble Space Telescope is the direct solution to a problem that telescopes have faced since the very earliest days of their invention: the atmosphere. The quandary is twofold: Shifting air pockets in Earth's atmosphere distort the view of telescopes on the ground, no matter how large or scientifically advanced those telescopes are. This "atmospheric distortion" is the reason that the stars seem to twinkle when you look up at the sky.

The atmosphere also partially blocks or absorbs certain wavelengths of radiation, like ultraviolet, gamma- and X-rays, before they can reach Earth. Scientists can best examine an object like a star by studying it in all the types of wavelengths that it emits.

Newer ground-based telescopes are using technological advances to try to correct atmospheric distortion, but there's no way to see the wavelengths the atmosphere prevents from even reaching the planet.

The most effective way to avoid the problems of the atmosphere is to place your telescope beyond it. Or, in Hubble's case, 353 miles (569 km) above the surface of Earth.

Every 97 minutes, Hubble completes a spin around Earth, moving at the speed of about five miles per second (8 km per second) — fast enough to travel across the United States in about 10 minutes. As it travels, Hubble's mirror captures light and directs it into its several science instruments.

Hubble is a type of telescope known as a Cassegrain reflector. Light hits the telescope's main mirror, or primary mirror. It bounces off the primary mirror and encounters a secondary mirror. The secondary mirror focuses the light through a hole in the center of the primary mirror that leads to the telescope's science instruments.

People often mistakenly believe that a telescope's power lies in its ability to magnify objects. Telescopes actually work by collecting more light than the human eye can capture on its own. The larger a telescope's mirror, the more light it can collect, and the better its vision. Hubble's primary mirror is 94.5 inches (2.4 m) in diameter. This mirror is small compared with those of current ground-based telescopes, which can be 400 inches (1,000 cm) and up, but Hubble's location beyond the atmosphere gives it remarkable clarity.

Once the mirror captures the light, Hubble's science instruments work together or individually to provide the observation. Each instrument is designed to examine the universe in a different way.

The Wide Field Camera 3 (WFC3) sees three different kinds of light: near-ultraviolet, visible and near-infrared, though not simultaneously. Its resolution and field of view are much greater than that of Hubble's other instruments. WFC3 is one of Hubble's two newest instruments, and will be used to study dark energy and dark matter, the formation of individual stars and the discovery of extremely remote galaxies previously beyond Hubble's vision.

The Cosmic Origins Spectrograph (COS), Hubble's other new instrument, is a spectrograph that sees exclusively in ultraviolet light. Spectrographs acts something like prisms, separating light from the cosmos into its component colors. This provides a wavelength "fingerprint" of the object being observed, which tells us about its temperature, chemical composition, density, and motion. COS will improve Hubble's ultraviolet sensitivity at least 10 times, and up to 70 times when observing extremely faint objects.

The Advanced Camera for Surveys (ACS) sees visible light, and is designed to study some of the earliest activity in the universe. ACS helps map the distribution of dark matter, detects the most distant objects in the universe, searches for massive planets, and studies the evolution of clusters of galaxies. ACS partially stopped working in 2007 due to an electrical short, but was repaired during Servicing Mission 4 in May 2009.

The Space Telescope Imaging Spectrograph (STIS) is a spectrograph that sees ultraviolet, visible and near-infrared light, and is known for its ability to hunt black holes. While COS works best with small sources of light, such as stars or quasars, STIS can map out larger objects like galaxies. STIS stopped working due to a technical failure on August 3, 2004, but was also repaired during Servicing Mission 4.

The Near Infrared Camera and Multi-Object Spectrometer (NICMOS) is Hubble's heat sensor. Its sensitivity to infrared light — perceived by humans as heat — lets it observe objects hidden by interstellar dust, like stellar birth sites, and gaze into deepest space.

Finally, the Fine Guidance Sensors (FGS) are devices that lock onto "guide stars" and keep Hubble pointed in the right direction. They can be used to precisely measure the distance between stars, and their relative motions.

All of Hubble's functions are powered by sunlight. Hubble sports solar arrays that convert sunlight directly into electricity. Some of that electricity is stored in batteries that keep the telescope running when it's in Earth's shadow, blocked from the Sun's rays.

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Seeing Infinity with the Hubble Telescope*

All telescopes have optical systems, and some even have specialized instruments. Hubble has additional requirements because it operates in space — the telescope is actually "flown" as a spacecraft. Therefore, several spacecraft support systems are in place to keep Hubble functioning smoothly in space. These systems girdle the body of the telescope.

Much like a robot or computer in space, Hubble performs only in response to detailed instructions from the people on the ground. Hubble has communications antennae so that astronomers and technicians can communicate with the telescope — telling it what to do and when to do it. Four antennae send and receive information between the telescope and the Flight Operations Team at Goddard Space Flight Center in Greenbelt, Md.

Scientists communicate with the telescope via the Tracking and Data Relay Satellite (TDRS) system. There are currently five TDRS satellites located at various locations in the sky.

In order for this system to work, at least one of the five satellites must be visible from the spacecraft's line of sight. Scientists can interact directly with the telescope during times of satellite visibility, allowing them to make small changes in the spacecraft pointing to fine-tune their observations.

Satellite visibility does not affect a planned observation because the commanding is done well in advance. When none of the satellites are visible from the spacecraft, a special data recorder stores the observation. The data are stored and then transmitted during periods of satellite visibility.

What gives Hubble such remarkable eyesight? What makes its pictures of distant objects so sharp? Its position above Earth's atmosphere — although clearly advantageous — is only part of the answer. Without powerful eyesight, Hubble would not be able to take full advantage of its unique location.

Hubble's "eyes" are actually a system called the Optical Telescope Assembly. That system consists of two mirrors, support trusses, and the apertures (openings) of the instruments. Hubble's optical system is a straightforward design known as Ritchey-Chretien Cassegrain, in which two special mirrors form focused images over the largest possible field of view.

Incoming light travels down a tube fitted with baffles that keep out stray light. The light is collected by the concave (curved inward, like a bowl) primary mirror and reflected toward the smaller, convex (curved outward, like a dome) secondary mirror. The secondary mirror bounces the light back toward the primary mirror and through a hole in its center. The light is then focused on a small area called the focal plane, where it is picked up by the various science instruments.

Hubble's mirrors are very smooth and have precisely shaped reflecting surfaces. They were ground (shaped by removing glass with abrasives) so that their surfaces do not deviate from a perfect curve by more than 1/800,000ths of an inch. If Hubble's primary mirror were scaled up to the diameter of the Earth, the biggest bump would be only six inches tall.

Shortly after Hubble's deployment in 1990, scientists found that the curve to which the primary mirror was ground was incorrect, causing "spherical aberration." Fortunately, corrective optics were able to solve this problem.

Hubble's mirrors are made of ultra-low expansion glass and kept at a nearly constant room temperature (about 70 degrees Fahrenheit) to avoid warping. The reflecting surfaces are coated with a 3/1,000,000th-inch layer of pure aluminum and protected by a 1/1,000,000th-inch layer of magnesium fluoride. The magnesium fluoride makes the mirrors more reflective of ultraviolet light.

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How Stars Get Their Names*

By Janette Vince

For thousands of years, human beings have used the night sky to navigate, keep track of the seasons, and inspire myths and legends. The tradition of naming stars is as old as history itself. Before modern times, however, humans could only name the stars that were visible in the night sky--a tiny fraction of the number of stars we can see today with powerful telescopes. Some stars have beautiful and evocative names, while some stars are designated by unimaginative-sounding groups of numbers and letters. So how do stars get their names?

Today, most stars are not given proper names. However, a few stars have kept names given many years ago. Here are a few ways a star may have come by its name.

Tradition
Some stars stand out from the rest. These "stars among stars" have been singled out with traditional names for centuries. Polaris, for example, is the one star that seems to occupy a fixed position in the heavens. People have been using it as a navigation aid for millennia, and it has had many different names in various cultures. In addition to Polaris, Western culture occasionally refers to it as the North Star or the Pole Star.

Ancient star catalogues
Some star names have been preserved in the works of ancient astronomers. Perhaps the earliest star catalogue we know of was written by Gan De, a Chinese astronomer who lived in the 4th century BC. The Western world's first star catalogue was written by Timocharis, an astronomer from Alexandria, about a hundred years later.

Most of the ancient star names still in use today, however, can be traced to the 2nd century AD. Ptolemy, a Greek mathematician and astronomer who lived in Egypt almost two thousand years ago, wrote a star catalogue in The Almagest, a mathematical and astronomical document outlining star and planetary motions and mechanics.

Ptolemy's catalogue contains over a thousand stars. Most of these are identified first by their position within a certain constellation; second by their longitude and latitude; and third by their magnitude, or brightness. He did give a few stars special names, most of which are in common use today. These include Arcturus, Sirius, Regulus, Capella, and Spica.

Medieval Arabic translations
In the Middle Ages, Ptolemy's Almagest was adopted by Arabic astronomers, who translated many of the original Greek names into Arabic. Most of the Arabic names were derived from Ptolemy's descriptions of the locations of the stars within their constellations. For example, Arab astronomers named a star within the left foot of Orion the Hunter "Rigel," which is Arabic for "foot." Other stars whose names derive from Arabic include Deneb, Betelgeuse, Vega, and Altair.

Prominent astronomers
A very few stars are named after the astronomers who studied them. Barnard's Star, for example, is a red dwarf named after E. E. Barnard, who discovered it in 1916. Van Maanen's Star is the second white dwarf star ever found, and it was named after Adrian Van Maanen, its discoverer. Bessel's Star is named after George Friedrich Bessel, who measured its distance from Earth in 1838.

Powerful people
Even more rarely, a star can be named after an important figure in history. For example, the brightest star in the Canes Venatici (Hunting Dogs) constellation is named Cor Caroli, meaning "Heart of Charles." Historians are not sure whether it was named in honor of King Charles I or King Charles II of England.

Bayer designations
During the early 17th century, German astronomer Johann Bayer traveled by ship to different hemispheres in search of stars to observe. Bayer compiled a star catalogue in which he named stars by designating first a lower-case Greek letter, such as alpha or gamma, and then the Latin name of the constellation each star could be found in. The Latin constellation names were usually given in the possessive form, to indicate the star "belonged" to that constellation. Many of these names are still in use today, including Alpha Centauri, Alpha Canis Majoris, and Beta Persei.

Modern sky catalogues
The situation gets a bit complicated when it comes to the way stars are named today. Astronomers are performing new sky surveys and compiling star catalogues to record new discoveries every day. Some of these catalogues are extremely large--the Guide Star Catalogue II, for example, contains over 998 million stars. There are too many stars to give each one a unique proper name. As a result, most naming conventions depend on a series of numbers indicating the star's location, brightness, and other factors. An example is SDSSp J153259.96-003944.1. The lettered section (SDSSp) indicates that the designation is from the Sloan Digital Sky Survey of preliminary objects, and the numbers give the star's location in the sky.

The stars we see when we look into the sky on a clear night are only a tiny fraction of the number we can see through a powerful telescope--and those in turn represent only a tiny amount of the total number of stars too far away to see. With the billions of stars in existence, it's not practical to give each one a special name of its own. That makes the few stars with proper names almost unique in the universe.

About the Author: J Vince is managing director of the E-Commerce London based experience days companyhttp://www.thanksdarling.com For more articles as well as a range of gifts where you can name your own star visit http://www.thanksdarling.com/categories/out-of-this-world.htm

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