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We live in an enormous "island" of stars called the Milky Way galaxy -- a collection of over 200 billion stars (and probably billions of planets). Since we are inside the Milky Way, it's hard to take a complete picture of it. It's like trying to take a picture of yourself from inside your gall bladder -- the view is not too clear. But we can learn about our own galaxy by observing others that are like us in size and shape.
In this beautiful picture, assembled from a Hubble Space Telescope image, as well as color information from a Japanese telescope and other observers, you can see a near cousin of the Milky Way located at a distance of about 50 million lightyears. (That means the light from this great "city of stars" took 50 million years to reach us!)
This galaxy doesn't have a name (like most galaxies), but simply referred to by a catalog number -- NGC 7331. Like our Milky Way, it is a spiral-shaped assemblage of stars and cosmic gas and dust. It contains about the same amount of material as our galaxy. We see it tilted to our line of sight, accounting for its oval (rather than circular) appearance.
It appears from other observations that NGC 7331 has a giant black hole at the center that has swallowed much more material than the black hole at the heart of the Milky Way. We are no longer surprised when we find giant black holes at the cores of galaxies -- it appears most grown-up galaxies have them.
Above and to the left of NGC 7331 are two other galaxies, which are about 10 times farther away! They just happen to be in the frame of the picture, like movie extras behind the actor whom our attention is focused on. Billions upon billions of galaxies make up the observable universe.
This gorgeous picture was assembled from different sources by Dr. Robert Gendler, a physician on the East Coast, whose passion is astronomical photography and whose skill at computer rendering of our skies is extra-ordinary. As many students and teachers travel or rest for spring vacation around this time of year, here is a celestial tourist sight for your enjoyment.
This past week, the scientists at the Space Telescope Science Institute released a remarkable picture, which combines Hubble images with those taken by advanced amateur astronomers on Earth, and I'm glad to be able to share it with you. (Click on the picture to make it bigger.)
In the image, we're seeing the central part of an "active" or disturbed galaxy about 25 million light years away, with the catalog number M106. You can see the yellow center of the galaxy, crowded with stars whose light blends together. You can also see its great spiral arms of stars, gently curving away from that center. They are outlined with blue stars and regions where gas is being heated by stars and glowing red. Those arms go much further out than the boundaries of this picture.
But what is especially interesting is that there are also great jets of glowing red gas, which are not part of the flat spiral structure of the galaxy. Instead, they are at odd angles to the galaxy's disk. What we are seeing there are great clouds of glowing hydrogen -- the most common element in all galaxies. But what makes them glow so intensely? Astronomers have good evidence that they are energized by radiation (not visible to the human eye) coming from a super-giant black hole at the center of the galaxy.
Neither the black hole (somewhere in the middle of the yellow center) or its rays of energy are visible here. But when those rays hit the gas that is the "raw material" of such galaxies, they make it glow with a fierce intensity.
This beautiful photograph was assembled by an amateur astronomer, Robert Gendler, a doctor living on the East Coast, who has made astronomical photography his hobby and his passion. It's quite a coup to have his photograph used and released by the Hubble scientists. For more of Dr. Gendler's photos, see his web site at: http://www.robgendlerastropics.com/
If you want to learn some basic information about black holes, you can check out a little video of a 6-minute talk I gave about them at the SETI Institute: http://www.youtube.com/watch?v=7DX_cc-IjpY
Amateur astrophotographer R. Jay Gabany also contributed to the image. You can see his image (with less detail) but showing the whole galaxy and really emphasizing the gas excited by the black hole at:
http://apod.nasa.gov/apod/image/1103/lrg_ngc4258gabany.jpg
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