Stellar Evolution

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years for the most massive to trillions of years for the least massive, which is considerably longer than the age of the universe. All stars are born from collapsing clouds of gas and dust, often called nebulae or molecular clouds. Over the course of millions of years, these protostars settle down into a state of equilibrium, becoming what is known as a main-sequence star.

Nuclear fusion powers a star for most of its life. Initially the energy is generated by the fusion of hydrogen atoms at the core of the main-sequence star. Later, as the preponderance of atoms at the core becomes helium, stars like the Sun begin to fuse hydrogen along a spherical shell surrounding the core. This process causes the star to gradually grow in size, passing through the subgiant stage until it reaches the red giant phase. Stars with at least half the mass of the Sun can also begin to generate energy through the fusion of helium at their core, whereas more massive stars can fuse heavier elements along a series of concentric shells. Once a star like the Sun has exhausted its nuclear fuel, its core collapses into a dense white dwarf and the outer layers are expelled as a planetary nebula. Stars with around ten or more times the mass of the Sun can explode in a supernova as their inert iron cores collapse into an extremely dense neutron star or black hole. Although the universe is not old enough for any of the smallest red dwarfs to have reached the end of their lives, stellar models suggest they will slowly become brighter and hotter before running out of hydrogen fuel and becoming low-mass white dwarfs.

Stellar evolution is not studied by observing the life of a single star, as most stellar changes occur too slowly to be detected, even over many centuries. Instead, astrophysicists come to understand how stars evolve by observing numerous stars at various points in their lifetime, and by simulating stellar structure using computer models.

Image Credit: unknown
Explanation from: http://en.wikipedia.org/wiki/Stellar_evolution

The Carina Nebula imaged by the VLT Survey Telescope

The spectacular star-forming Carina Nebula has been captured in great detail by the VLT Survey Telescope at ESO’s Paranal Observatory. This picture was taken with the help of Sebastián Piñera, President of Chile, during his visit to the observatory on 5 June 2012 and released on the occasion of the new telescope’s inauguration in Naples on 6 December 2012.

Image Credit: ESO
Explanation from: http://www.eso.org/public/images/eso1250a/

The Horsehead Nebula

A reproduction of a composite colour image of the Horsehead Nebula and its immediate surroundings. It is based on three exposures in the visual part of the spectrum with the FORS2 multi-mode instrument at the 8.2-m KUEYEN telescope at Paranal. It was produced from three images, obtained on February 1, 2000, with the FORS2 multi-mode instrument at the 8.2-m KUEYEN Unit Telescope and extracted from the VLT Science Archive Facility . The frames were obtained in the B-band (600 sec exposure; wavelength 429 nm; FWHM 88 nm; here rendered as blue), V-band (300 sec; 554 nm; 112 nm; green) and R-band (120 sec; 655 nm; 165 nm; red). The original pixel size is 0.2 arcsec. The photo shows the full field recorded in all three colours, approximately 6.5 x 6.7 arcmin 2 . The seeing was about 0.75 arcsec.

Image Credit: ESO
Explanation from: http://www.eso.org/public/images/eso0202a/

NGC 2467 and Surroundings

Area surrounding the stellar cluster NGC 2467, located in the southern constellation of Puppis ("The Stern"). With an age of a few million years at most, it is a very active stellar nursery, where new stars are born continuously from large clouds of dust and gas. The image, looking like a colourful cosmic ghost or a gigantic celestial Mandrill, contains the open clusters Haffner 18 (centre) and Haffner 19 (middle right: it is located inside the smaller pink region — the lower eye of the Mandrill), as well as vast areas of ionised gas. The bright star at the centre of the largest pink region on the bottom of the image is HD 64315, a massive young star that is helping shaping the structure of the whole nebular region.

Image Credit: ESO
Explanation from: http://www.eso.org/public/images/eso0544a/

A 340-million pixel starscape from Paranal

The second of three images of ESO’s GigaGalaxy Zoom project is a new and wonderful 340-million-pixel vista of the central parts of our galactic home, a 34 by 20-degree wide image that provides us with a view as experienced by amateur astronomers around the world. Taken by Stéphane Guisard, an ESO engineer and world-renowned astrophotographer, from Cerro Paranal, home of ESO’s Very Large Telescope, this second image directly benefits from the quality of Paranal’s sky, one of the best on the planet. The image shows the region spanning the sky from the constellation of Sagittarius (the Archer) to Scorpius (the Scorpion). The very colourful Rho Ophiuchi and Antares region features prominently to the right, as well as much darker areas, such as the Pipe and Snake Nebulae. The dusty lane of our Milky Way runs obliquely through the image, dotted with remarkable bright, reddish nebulae, such as the Lagoon and the Trifid Nebulae, as well as NGC 6357 and NGC 6334. This dark lane also hosts the very centre of our Galaxy, where a supermassive black hole is lurking. The image was obtained by observing with a 10-cm Takahashi FSQ106Ed f/3.6 telescope and a SBIG STL CCD camera, using a NJP160 mount. Images were collected through three different filters (B, V and R) and then stitched together. This mosaic was assembled from 52 different sky fields made from about 1200 individual images totalling 200 hours exposure time, with the final image having a size of 24 403 x 13 973 pixels. Note that the final, full resolution image is only available through Stéphane Guisard.

Image Credit: ESO/S. Guisard
Explanation from: http://www.eso.org/public/images/eso0934a/

VST image of the star-forming region Messier 17

The first released VST image shows the spectacular star-forming region Messier 17, also known as the Omega Nebula or the Swan Nebula, as it has never been seen before. This vast region of gas, dust and hot young stars lies in the heart of the Milky Way in the constellation of Sagittarius (The Archer). The VST field of view is so large that the entire nebula, including its fainter outer parts, is captured — and retains its superb sharpness across the entire image. The data were processed using the Astro-WISE software system developed by E.A. Valentijn and collaborators at Groningen and elsewhere.

Image Credit: ESO/INAF-VST/OmegaCAM, OmegaCen/Astro-WISE/Kapteyn Institute
Explanation from: http://www.eso.org/public/images/eso1119a/

VISTA's infrared view of the Orion Nebula

This wide-field view of the Orion Nebula (Messier 42), lying about 1350 light-years from Earth, was taken with the VISTA infrared survey telescope at ESO’s Paranal Observatory in Chile. The new telescope’s huge field of view allows the whole nebula and its surroundings to be imaged in a single picture and its infrared vision also means that it can peer deep into the normally hidden dusty regions and reveal the curious antics of the very active young stars buried there. This image was created from images taken throughZ, J and Ks filters in the near-infrared part of the spectrum. The exposure times were ten minutes per filter. The image covers a region of sky about one degree by 1.5 degrees.

Image Credit: ESO/J. Emerson/VISTA./Acknowledgment: Cambridge Astronomical Survey Unit
Explanation from: http://www.eso.org/public/images/eso1006a/

The Very Large Telescope Snaps a Stellar Nursery and Celebrates Fifteen Years of Operations

This intriguing new view of a spectacular stellar nursery IC 2944 is being released to celebrate a milestone: 15 years of ESO’s Very Large Telescope. This image also shows a group of thick clouds of dust known as the Thackeray globules silhouetted against the pale pink glowing gas of the nebula. These globules are under fierce bombardment from the ultraviolet radiation from nearby hot young stars. They are both being eroded away and also fragmenting, rather like lumps of butter dropped onto a hot frying pan. It is likely that Thackeray’s globules will be destroyed before they can collapse and form new stars.

Image Credit: ESO
Explanation from: http://www.eso.org/public/images/eso1322a/

The View from the Top

There have been many images of the full disc of Earth from space—a view often referred to as “the Blue Marble”—but few have looked quite like this. Using natural-color images from the Visible/Infrared Imaging Radiometer Suite (VIIRS) on the recently launched Suomi-NPP satellite, a NASA scientist has compiled a new view showing the Arctic and high latitudes.

Ocean scientist Norman Kuring of NASA’s Goddard Space Flight Center pieced together this composite image of Europe, Asia, North Africa, and the entire Arctic. It was compiled from 15 satellite passes made by Suomi-NPP on May 26, 2012. The spacecraft circles the Earth from pole to pole at an altitude of 824 kilometers (512 miles), so it takes multiple passes to gather enough data to show an entire hemisphere without gaps in the view.

Kuring stitched the image swaths together and then set up this view looking down from 70 degrees North, 60 degrees East. (That is, the view is artificial, as the satellite does not see the full disc at one time.) He was able to show the Arctic in this image because northern hemisphere spring spreads enough sunlight over the North Pole to allow a natural-light view.

VIIRS is a scanning radiometer that acquires data in 22 spectral bands, covering visible, near-infrared, and thermal infrared regions of the electromagnetic spectrum. It was designed to extend and improve upon the measurements of land masses, oceans, ice, and the atmosphere made over the past two decades by the Advanced Very High Resolution Radiometer (AVHRR) and the Moderate Resolution Imaging Spectroradiometer (MODIS).

Image Credit: Norman Kuring, NASA/GSFC/Suomi NPP
Explanation from: http://earthobservatory.nasa.gov/IOTD/view.php?id=78349

Most Amazing High Definition Image of Earth - Blue Marble 2012

A 'Blue Marble' image of the Earth taken from the VIIRS instrument aboard NASA's Earth-observing satellite - Suomi NPP. This composite image uses a number of swaths of the Earth's surface taken on January 4, 2012. The NPP satellite was renamed 'Suomi NPP' on January 24, 2012 to honor the late Verner E. Suomi of the University of Wisconsin.

Suomi NPP is NASA's next Earth-observing research satellite. It is the first of a new generation of satellites that will observe many facets of our changing Earth.

Suomi NPP is carrying five instruments on board. The biggest and most important instrument is The Visible/Infrared Imager Radiometer Suite or VIIRS.

Image Credit: NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring
Explanation from: http://www.nasa.gov/multimedia/imagegallery/image_feature_2159.html

Earth - Behind the Scenes

 

People love images of the full disk of Earth as it would appear from space. In the last week of January 2012, one of our NASA science colleagues, Norman Kuring, took some fresh data from the newest Earth-observing satellite, projected it on a disk, and voila...three million people viewed the image in a week. The wave of interest is still spreading across the Internet and social media.

Have you ever considered how these images get made? The image above, of Earth’s eastern hemisphere, was built from data collected by the Suomi NPP satellite, which flies in a polar orbit at an altitude of 824 kilometers (512 miles). The perspective of this new image, however, is from 12,743 kilometers (7,918 miles) above a point at 10 degrees South latitude and 45 degrees East longitude.

Kuring managed to “step back” from Earth by combining data from six separate orbits Suomi NPP made on January 23, 2012. The natural-color images come from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the satellite. The four vertical lines of haze are caused by sunglint, the reflection of sunlight off the ocean.

The graphic above illustrates how separate images in red, green, and blue wavelengths are combined to make natural-color imagery, and how multiple, adjacent swaths—slices of Earth viewed on each satellite pass—get built into a composite. The width of the swaths covered by each pass of VIIRS is about 3,001 kilometers (1,865 miles).

A decade ago, members of the Earth Observatory team pulled together about 10,000 satellite scenes collected over 100 days, and created a 43,200-pixel by 21,600-pixel map of the Earth. By 2007, the resolution of this full Earth “Blue Marble” was up to 86,400 pixels by 43,200 pixels. Even after all of these years, the Blue Marble is still the most popular and most downloaded image on our site.

Of course, the original Blue Marble image is actually a photograph—not a composite view. It was taken by the Apollo 17 astronauts as they traveled to the Moon. The view is from a distance of about 45,000 kilometers (28,000 miles).

Image Credit: NASA/NOAA

Explanation from: http://earthobservatory.nasa.gov/IOTD/view.php?id=77085

The United States at night seen by Suomi NPP satellite

This image of the continental United States at night is a composite assembled from data acquired by the Suomi NPP satellite in April and October 2012. The image was made possible by the satellite's "day-night band" of the Visible Infrared Imaging Radiometer Suite (VIIRS), which detects light in a range of wavelengths from green to near-infrared and uses filtering techniques to observe dim signals such as city lights, gas flares, auroras, wildfires and reflected moonlight.

Image Credit: NASA Earth Observatory/NOAA NGDC
Explanation from: http://www.nasa.gov/mission_pages/NPP/news/earth-at-night.html

The Majestic Sombrero Galaxy (M104)

NASA's Hubble Space Telescope has trained its razor-sharp eye on one of the universe's most stately and photogenic galaxies, the Sombrero galaxy, Messier 104 (M104). The galaxy's hallmark is a brilliant white, bulbous core encircled by the thick dust lanes comprising the spiral structure of the galaxy. As seen from Earth, the galaxy is tilted nearly edge-on. We view it from just six degrees north of its equatorial plane. This brilliant galaxy was named the Sombrero because of its resemblance to the broad rim and high-topped Mexican hat.

At a relatively bright magnitude of +8, M104 is just beyond the limit of naked-eye visibility and is easily seen through small telescopes. The Sombrero lies at the southern edge of the rich Virgo cluster of galaxies and is one of the most massive objects in that group, equivalent to 800 billion suns. The galaxy is 50,000 light-years across and is located 28 million light-years from Earth.

Hubble easily resolves M104's rich system of globular clusters, estimated to be nearly 2,000 in number — 10 times as many as orbit our Milky Way galaxy. The ages of the clusters are similar to the clusters in the Milky Way, ranging from 10-13 billion years old. Embedded in the bright core of M104 is a smaller disk, which is tilted relative to the large disk. X-ray emission suggests that there is material falling into the compact core, where a 1-billion-solar-mass black hole resides.

In the 19th century, some astronomers speculated that M104 was simply an edge-on disk of luminous gas surrounding a young star, which is prototypical of the genesis of our solar system. But in 1912, astronomer V. M. Slipher discovered that the hat-like object appeared to be rushing away from us at 700 miles per second. This enormous velocity offered some of the earliest clues that the Sombrero was really another galaxy, and that the universe was expanding in all directions.

The Hubble Heritage Team took these observations in May-June 2003 with the space telescope's Advanced Camera for Surveys. Images were taken in three filters (red, green, and blue) to yield a natural-color image. The team took six pictures of the galaxy and then stitched them together to create the final composite image. One of the largest Hubble mosaics ever assembled, this magnificent galaxy has an apparent diameter that is nearly one-fifth the diameter of the full moon.

Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Explanation from: http://hubblesite.org/newscenter/archive/releases/2003/28/image/a/

Sunset over Western South America

Astronauts onboard the International Space Station see, on average, 16 sunrises and sunsets during a 24-hour orbital period. Each changeover between day and night is marked by the terminator, a line on Earth's surface separating the sunlit side from the darkness.

While the terminator is often conceptualized as a hard boundary—and is frequently presented as such in graphics and visualizations—in reality the edge of light and dark is diffuse due to the scattering of light by the Earth’s atmosphere. This zone of diffuse lighting is experienced as dusk or twilight on the ground; while the Sun is no longer visible, some illumination is still present due to light scattering over the local horizon.

The terminator is visible in this panoramic view across central South America, looking towards the northeast. An astronaut shot the photo at approximately 7:37 p.m. local time. Layers of the Earth’s atmosphere, colored bright white to deep blue, are visible on the horizon (or limb). The highest cloud tops have a reddish glow due to direct light from the setting Sun, while lower clouds are in twilight.

The Salar de Coipasa, a large salt lake in Bolivia, is dimly visible on the night side of the terminator. The salar provides a geographic reference point for determining the location and viewing orientation of the image.

Image Credit: NASA
Explanation from: http://earthobservatory.nasa.gov/IOTD/view.php?id=50205

The crescent Moon and earthshine over ESO's Paranal Observatory

This view shows the thin crescent Moon setting over ESO’s Paranal Observatory in Chile. As well as the bright crescent the rest of the disc of the Moon can be faintly seen. This phenomenon is called earthshine. It is due to sunlight reflecting off the Earth and illuminating the lunar surface. By observing earthshine astronomers can study the properties of light reflected from Earth as if it were an exoplanet and search for signs of life.

This picture was taken on 27 October 2011 and also records the planets Mercury and Venus

Image Credit: ESO/B. Tafreshi

Explanation of the video from: http://www.eso.org/public/images/eso1210a/

To Fly Free in Space above Home

At about 100 meters from the cargo bay of the space shuttle Challenger, Bruce McCandless II was farther out than anyone had ever been before. Guided by a Manned Maneuvering Unit (MMU), astronaut McCandless, on this picture, was floating free in space. McCandless and fellow NASA astronaut Robert Stewart were the first to experience such an "untethered space walk" during Space Shuttle mission 41-B in 1984. The MMU works by shooting jets of nitrogen and has since been used to help deploy and retrieve satellites. With a mass over 140 kilograms, an MMU is heavy on Earth, but, like everything, is weightless when drifting in orbit. The MMU was replaced with the SAFER backpack propulsion unit.

Image Credit: STS-41B, NASA
Explanation from: http://apod.nasa.gov/apod/ap120101.html

The Solstice Moon's Eclipse

A big, bright, beautiful Full Moon slid into planet Earth's shadow early Tuesday on December 21, 2010 morning. Remarkably, the total lunar eclipse coincided with the date of the Winter Solstice. During the eclipse, the best viewing in North America found the coppery lunar disc high in a cold winter sky, the Moon reddened by light filtering into the Earth's dark central shadow or umbra. The light comes from all the sunsets and sunrises, seen from a lunar perspective around the edges of a silhouetted Earth. Passing closer to the center of the umbra, the Moon's southern hemisphere (left) appears darker in this eclipse image, recorded from Deerlick Astronomy Village, Georgia, USA. The picture is a digital composite, a separate longer exposure added to an eclipse frame to capture the surrounding star field.

Image Credit & Copyright: Chris Hetlage
Explanation from: http://apod.nasa.gov/apod/ap101223.html

Solar Eclipse over Queensland

In 2012 New Moon brought a total solar eclipse to parts of planet Earth on November 13 (UT). Most of the total eclipse track fell across the southern Pacific, but the Moon's dark umbral shadow began its journey in northern Australia on Wednesday morning, local time. From along the track, this telescopic snapshot captures the Moon's silhouette in skies over Queensland along the Mulligan highway west of Port Douglas. Almost completely covered, the Sun's disk is seen still surrounded by a hint of the faint solar corona. Planet-sized prominences stretch above the active Sun's edge. Sunlight streaming through gaps in the rugged profile of the lunar limb creates the brilliant but fleeting Baily's Beads.

Image Credit & Copyright: Phil Hart
Explanation from: http://apod.nasa.gov/apod/ap121115.html

Kepler-22b: Closer to Finding an Earth

This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the habitable zone of a sun-like star. It is the first planet that NASA's Kepler Space Telescope mission has confirmed to orbit in a star's habitable zone - the region around a star where liquid water, a requirement for life on Earth, could persist. The planet is 2.4 times the size of Earth, making it the smallest yet found to orbit in the middle of the habitable zone of a star like our Sun.

Scientists do not yet know if the planet has a predominantly rocky, gaseous or liquid composition. It's possible that the world would have clouds in its atmosphere, as depicted here in the artist's interpretation.

Image Credit: NASA/Ames/JPL-Caltech
Explanation from: http://www.nasa.gov/mission_pages/kepler/multimedia/images/kepler-22b.html

Home from Above

There's no place like home. Peering out of the windows of the *+International Space Station* (ISS), astronaut Tracy Caldwell Dyson takes in the planet on which we were all born, and to which she would soon return. About 350 kilometers up, the ISS is high enough so that the Earth's horizon appears clearly curved. Astronaut Dyson's windows show some of Earth's complex clouds, in white, and life giving atmosphere and oceans, in blue. The space station orbits the Earth about once every 90 minutes. This image was taken in late September 2010 from the ISS's Cupola window.

Image Credit: NASA
Explanation from: http://apod.nasa.gov/apod/ap101115.html

Discovery's Dawn

On April 5, visitors to Kennedy Space Center saw these colorful clouds, twisting and drifting through dawn skies. Of course, the clouds were rocket engine plumes from the predawn launch of the space shuttle Discovery on the STS-131 mission to the International Space Station. Their layered colors are created as they reflect the reddened light from the still rising Sun. Fittingly, denizens of the space center's rocket garden are lit in the foreground. At the far left is a 1960s vintage multistage Atlas-Agena rocket. Together on the right, are Mercury-Redstone and Mercury-Atlas rockets.

Image Credit & Copyright: Craig Crawford
Explanation: http://apod.nasa.gov/apod/ap100409.html

Composite map of the world assembled from data acquired by the Suomi NPP satellite

Sun-synchronous orbit (824 km above the Earth)

April and October 2012

Image Credit: NASA Earth Observatory/NOAA NGDC

Chance Alignment Between Galaxies Mimics a Cosmic Collision

NASA's Hubble Space Telescope shows a rare view of a pair of overlapping galaxies, called NGC 3314. The two galaxies look as if they are colliding, but they are actually separated by tens of millions of light-years, or about ten times the distance between our Milky Way and the neighboring Andromeda galaxy. The chance alignment of the two galaxies, as seen from Earth, gives a unique look at the silhouetted spiral arms in the closer face-on spiral, NGC 3314A.

The motion of the two galaxies indicates that they are both relatively undisturbed and that they are moving in markedly different directions. This indicates they are not on any collision course. NGC 3314A's warped shape is likely due to an encounter with another nearby galaxy, perhaps the large spiral galaxy NGC 3312 (located outside the Hubble image).

Because of the alignment, NGC 3314B's dust lanes appear lighter than those of NGC 3314A. This is not because that galaxy lacks dust, but rather because its dust lanes are lightened by the bright fog of stars in the foreground. NGC 3314A's dust, in contrast, is backlit by the stars of NGC 3314B, silhouetting them against the bright background.

The color composite was produced from exposures taken in blue and red light with Hubble's Advanced Camera for Surveys. The pair of galaxies lie roughly 140 million light-years from Earth, in the direction of the southern hemisphere constellation Hydra.

Image Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration, and W. Keel (University of Alabama)
Explanation from: http://hubblesite.org/newscenter/archive/releases/2012/29/image/a/

Annular Solar Eclipse seen from Kanyakumari

Kanyakumari, India

January 15, 2010

Image Credit & Copyright: Mikael Svalgaard

A Picturesque Venus Transit

The rare transit of Venus across the face of the Sun in 2004 was one of the better-photographed events in sky history. Both scientific and artistic images flooded in from the areas that could see the transit: Europe and much of Asia, Africa, and North America. Scientifically, solar photographers confirmed that the black drop effect is really better related to the viewing clarity of the camera or telescope than the atmosphere of Venus. Artistically, images might be divided into several categories. One type captures the transit in front of a highly detailed Sun. Another category captures a double coincidence such as both Venus and an airplane simultaneously silhouetted, or Venus and the International Space Station in low Earth orbit. A third image type involves a fortuitous arrangement of interesting looking clouds, as shown by example in this taken from North Carolina, USA. The next transit of Venus across the Sun will be on 10/11 December 2117.

Image Credit & Copyright: David Cortner
Explanation from: http://apod.nasa.gov/apod/ap111016.html

Hubble's Largest Galaxy Portrait Offers a New High-Definition View

Giant galaxies weren't assembled in a day. Neither was this Hubble Space Telescope image of the face-on spiral galaxy Messier 101 (M101). It is the largest and most detailed photo of a spiral galaxy that has ever been released from Hubble. The galaxy's portrait is actually composed of 51 individual Hubble exposures, in addition to elements from images from ground-based photos. The final composite image measures a whopping 16,000 by 12,000 pixels.

The Hubble archived observations that went into assembling this image were originally acquired for a range of Hubble projects: determining the expansion rate of the universe, studying the formation of star clusters in the giant star birth regions, finding the stars responsible for intense X-ray emission, and discovering blue supergiant stars.

The giant spiral disk of stars, dust, and gas is 170,000 light-years across or nearly twice the diameter of our galaxy, the Milky Way. M101 is estimated to contain at least one trillion stars. Approximately 100 billion of these stars could be like our Sun in terms of temperature and lifetime.

The galaxy's spiral arms are sprinkled with large regions of star-forming nebulae. These nebulae are areas of intense star formation within giant molecular hydrogen clouds. Brilliant young clusters of hot, blue, newborn stars trace out the spiral arms. The disk of M101 is so thin that Hubble easily sees many more distant galaxies lying behind the galaxy.

M101 (also nicknamed the Pinwheel Galaxy) lies in the northern circumpolar constellation, Ursa Major (The Great Bear), at a distance of 25 million light-years from Earth. Therefore, we are seeing the galaxy as it looked 25 million years ago — when the light we're receiving from it now was emitted by its stars — at the beginning of Earth's Miocene Period, when mammals flourished and the Mastodon first appeared on Earth. The galaxy fills a region in the sky equal to one-fifth the area of the full moon.

The newly composed image was assembled from Hubble archived images taken with the Advanced Camera for Surveys and the Wide Field and Planetary Camera 2 over nearly 10 years: in March 1994, September 1994, June 1999, November 2002, and January 2003. The Hubble exposures have been superimposed onto ground-based images, visible at the edge of the image, taken at the Canada-France-Hawaii Telescope in Hawaii, and at the 0.9-meter telescope at Kitt Peak National Observatory, part of the National Optical Astronomy Observatory in Arizona. The final color image was assembled from individual exposures taken through blue, green, and red (infrared) filters.

Image Credit: NASA, ESA, K. Kuntz (JHU), F. Bresolin (University of Hawaii), J. Trauger (Jet Propulsion Lab), J. Mould (NOAO), Y.-H. Chu (University of Illinois, Urbana), and STScI, Canada-France-Hawaii Telescope/ J.-C. Cuillandre/Coelum, G. Jacoby, B. Bohannan, M. Hanna/ NOAO/AURA/NSF
Explanation from: http://hubblesite.org/newscenter/archive/releases/2006/10/image/a/