Hubble's Galaxy Discoveries
Our Sun is just one of a vast number of stars within a galaxy called the Milky Way, which in turn is only one of the billions of galaxies in our universe. These massive cosmic neighborhoods, made up of stars, dust, and gas held together by gravity, come in a variety of sizes, from dwarf galaxies containing as few as 100 million stars to giant galaxies of more than a trillion stars. Astronomers generally classify galaxies into three major categories: spiral – like our Milky Way – elliptical, and irregular.
Astronomers quickly realized that Hubble had a flaw. Its mirror was slightly the wrong shape, causing the light that bounced off the center of the mirror to focus in a different place than light bouncing off the edge. This “spherical aberration,” about 1/50th the thickness of a sheet of paper, was corrected during the first servicing mission in 1993 with installation of the Corrective Optics Space Telescope Axial Replacement (COSTAR). The result was highresolution imaging as shown in the image of galaxy M100. Since then, all of Hubble’s instruments have had corrective optics built in, eventually making COSTAR unnecessary. It was removed from the telescope in 2009.
Hubble was upgraded four more times with improved instruments. The inset image is from Servicing Mission 1 (STS-61, Space Shuttle Endeavor) which took place in December 1993. Astronauts installed COSTAR and replaced Wide-Field Planetary Camera 1 (WFPC1) with Wide-Field Planetary Camera 2 (WFPC2), the first instrument to have the correction built into its optics. The image shows astronauts replacing WFPC1 with WFPC2.
Detailed note: The two images of the center of galaxy Messier 100 show WFPC1 and WFPC2 data and demonstrate how well Servicing Mission 1 corrected the mirror flaw. Hubble could now achieve its design specifications.
The largest Hubble Space Telescope image ever assembled, this sweeping view of a portion of the Andromeda galaxy (M31) is the sharpest large composite image ever taken of our galactic neighbor. Though the galaxy is over 2 million light-years away, Hubble is powerful enough to resolve individual stars in a 61,000-light-year-long stretch of the galaxy. The Andromeda galaxy is only 2.5 million light-years from Earth, making it a much bigger target in the sky than the myriad galaxies Hubble routinely photographs that are billions of light-years away.
The Hubble survey is assembled into a mosaic image using 7,398 exposures taken over 411 individual pointings. The data were taken with the Advanced Camera for Surveys. The lower left inset points out the numerous types of objects seen in the image. The lower right inset is a composite made from a series of ground observations that shows the entire M31 galaxy and the portion imaged by Hubble.
This 91-million pixel mosaic of the Whirlpool Galaxy (M51) was released to celebrate Hubble’s 15th anniversary. Beyond the sheer beauty of the image, the details along the spiral arms follow the progression of star formation from dark dust clouds through pink star-forming regions to blue newborn star clusters. Some astronomers believe that the Whirlpool's arms are so prominent because of the effects of a close encounter with NGC 5195, the small, yellowish galaxy at the outermost tip of one of the Whirlpool's arm.
The distance to M51 is 23 million light years (7 megaparsecs).
This image of the Sombrero Galaxy is one of the first large mosaics produced from the Advanced Camera for Surveys instrument. Combining data from six pointings, the full resolution image contains over 70 million pixels. The Sombrero is cataloged as 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. Sombrero is 28 million light years (9 megaparsecs) away.
These two spiral galaxies started to interact a few hundred million years ago, making the Antennae galaxies one of the nearest and youngest examples of a pair of colliding galaxies. Nearly half of the faint objects in the Antennae image are young clusters containing tens of thousands of stars.
The orange blobs to the left and right of image center are the two cores of the original galaxies and consist mainly of old stars criss-crossed by filaments of dust, which appear brown in the image. The two galaxies are dotted with brilliant blue star-forming regions surrounded by glowing hydrogen gas, appearing in the image in pink. The image allows astronomers to better distinguish between the stars and super star clusters created in the collision of two spiral galaxies. The Antennae are 62 million light years (19 megaparsecs) away.
Galaxy interactions are not always the grand collisions seen in the Antennae galaxies. These two interacting galaxies, called the Rose Galaxy or catalog name Arp 273, have produced less pronounced distortions in each others’ shape. The larger of the spiral galaxies, known as UGC 1810, has a disk that is tidally distorted into a rose-like shape by the gravitational tidal pull of the companion galaxy below it, known as UGC 1813. A swath of blue jewels across the top is the combined light from clusters of intensely bright and hot young blue stars. These massive stars glow fiercely in ultraviolet light. The smaller, nearly edge-on companion shows distinct signs of intense star formation at its nucleus, perhaps triggered by the encounter with the companion galaxy.
Some called this picture a “rose” of galaxies, with the upper galaxy as the bloom, and the lower galaxy as the stem. The pair is 340 million light years (105 megaparsecs) away.