Explanation: The Whirlpool Galaxy is a classic spiral galaxy. At only 30 million light years distant and fully 60 thousand light years across, M51, also known as NGC 5194, is one of the brightest and most picturesque galaxies on the sky. The above image is a digital combination of a ground-based image from the 0.9-meter telescope at Kitt Peak National Observatory and a space-based image from the Hubble Space Telescope, highlighting sharp features normally too red to be seen. Anyone with a good pair of binoculars, however, can see this Whirlpool toward the constellation of Canes Venaciti. M51 is a spiral galaxy of type Sc and is the dominant member of a whole group of galaxies. Astronomers speculate that M51's spiral structure is primarily due to its gravitational interaction with a smaller galaxy just off the top of this image.
Explanation: Is our Milky Way Galaxy surrounded by a halo of hot gas? A step toward solving this long-standing mystery was taken recently with Chandra X-ray observations of nearby galaxy NGC 4631. In the above composite picture, newly resolved diffuse X-ray emission is shown in blue, superposed on an HST image showing massive stars in red. Since NGC 4631 is similar to the Milky Way, this observation indicates that our own Galaxy is indeed surrounded by a halo of hot X-ray emitting gas, although we are too close to clearly differentiate them from more nearby extended X-ray sources. The clusters of massive stars probably heat the halo gas. Exactly how this gas gets ejected into a halo is a topic of continuing research.
Explanation: How did spiral galaxy ESO 510-13 get bent out of shape? The disks of many spirals are thin and flat, but not solid. Spiral disks are loose conglomerations of billions of stars and diffuse gas all gravitationally orbiting a galactic center. A flat disk is thought to be created by sticky collisions of large gas clouds early in the galaxy's formation. Warped disks are not uncommon, though, and even our own Milky Way Galaxy is thought to have a small warp. The causes of spiral warps are still being investigated, but some warps are thought to result from interactions or even collisions between galaxies. ESO 510-13, pictured above, is about 150 million light years away and about 100,000 light years across.
Explanation: The brightest galaxy visible from our own Milky Way Galaxy is the Large Magellanic Cloud (LMC). Visible predominantly from Earth's Southern Hemisphere, the LMC is the second closest galaxy, neighbor to the Small Magellanic Cloud, and one of eleven known dwarf galaxies that orbit our Milky Way Galaxy. The LMC is an irregular galaxy composed of a bar of older red stars, clouds of younger blue stars, and a bright red star forming region visible near the top of the above image called the Tarantula Nebula. The brightest supernova of modern times, SN1987A, occurred in the LMC.
Explanation: The party is still going on in spiral galaxy NGC 3310. Roughly 100 million years ago, NGC 3310 likely collided with a smaller galaxy causing the large spiral galaxy to light up with a tremendous burst of star formation. The changing gravity during the collision created density waves that compressed existing clouds of gas and triggered the star-forming party. The above image composite by the Hubble Space Telescope was used to find the ages of many of the resulting clusters of stars. To the surprise of many, some of the clusters are quite young, indicating that starburst galaxies may remain in star-burst mode for quite some time. NGC 3310 spans about 50,000 light years, lies about 50 million light years away, and is visible with a small telescope towards the constellation of Ursa Major.
Explanation: Spiral galaxy M33 is a mid-sized member of our Local Group of Galaxies. M33 is also called the Triangulum Galaxy for the constellation in which it resides. About four times smaller (in radius) than our Milky Way Galaxy and the Andromeda Galaxy (M31), it is much larger than the many of the local dwarf spheroidal galaxies. M33's proximity to M31 causes it to be thought by some to be a satellite galaxy of this more massive galaxy. M33's proximity to our Milky Way Galaxy causes it to appear more than twice the angular size of the Full Moon, and be visible with a good pair of binoculars. The above high-resolution image highlights light emitted by hydrogen in red and oxygen in blue. It was taken to help separate stars from emission nebulae, and therefore help study how galaxies form stars.
Explanation: If not perfect, then this spiral galaxy is at least one of the most photogenic. An island universe of about 100 billion stars, 30 million light-years away toward the constellation Pisces, NGC 628 or M74 presents a gorgeous face-on view to earthbound astronomers. Classified as an Sc galaxy, the grand design of M74's graceful spiral arms traced by bright blue star clusters and dark cosmic dust lanes, is similar in many respects to our own home galaxy, the Milky Way. Recorded with a 28 million pixel detector array, this impressive image celebrates first light for the Gemini Multi-Object Spectrograph (GMOS), a state-of-the-art instrument now operational at the 8-meter Gemini North telescope. The Gemini North Observatory gazes into the skies above Mauna Kea, Hawaii, USA, while its twin observatory, Gemini South, is scheduled to begin operations later this year from Cerro Pachón in central Chile.
Explanation: Do spiral galaxies look the same in every color? NGC 6782 demonstrates colorfully that they do not. In visible light, NGC 6782 appears to be a normal spiral galaxy with a bright bar across its center. In ultraviolet light, however, the central region blossoms into a spectacular and complex structure highlighted by a circumnuclear ring, as shown in the above representative color Hubble Space Telescope image. Many of the young stars that formed in a recent burst of star formation emit the ultraviolet light. Astronomers are studying possible relationships between the central bar and the ring. Light we see today from NGC 6782 left about 180 million years ago, while dinosaurs roamed the Earth. The galaxy spans about 80,000 light-years and can be seen with a telescope toward the constellation of Pavo.