Galaxies and How to Observe Them (Astronomers' Observing Guides)
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US state, Canadian province, or country. Tonight's Sky — Select location. Tonight's Sky — Enter coordinates. UTC Offset:. Giant volcano on Jupiter's moon Io could erupt any second Hubble spots a dim dark matter-rich galaxy Colliding asteroids may have caused an ancient ice age. The Sky This Week from September 20 to Picture of the Day Image Galleries. Learn about the Moon in a great new book. Dave's Universe Year of Pluto. Groups Why Join? Astronomy Day. Astronomy's Calendar. By Michael E. The Pinwheel Galaxy M33 lies in Triangulum.
Although its magnitude is a worthy 5.
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That makes its surface brightness low. Take your time, and switch to a high-power eyepiece to study the concentrations along its spiral arms. Unlike some deep-sky objects, galaxies can be challenging to see. Their light is spread out, and often their details are faint. Nonetheless, beginning sky observers might wonder how objects composed of up to a trillion or more individual suns could be so difficult to see.
Of course, the answer is distance. Galaxies are so far away that, except for a few, they appear small and faint. M82 has a greater surface brightness than most galaxies. To illustrate what this means, compare M82 with M81 in an eyepiece that just frames them both. Although M82 shines a magnitude and a half fainter than M81, it appears about as bright. Surface brightness is expressed in terms of magnitude per square arcminute, or per square arcsecond for small galaxies. Before you observe In my experience, six factors will determine how well your galaxy observing session goes.
The easternmost arm appears brighter. The core is small and round, and the bar extends to the northeast and southwest. Its central region is a maelstrom of star formation. Huge clumps of stars, visible through inch or larger telescopes, lie all along the spiral arms. Galaxy clusters Do you have a large scope and lots of patience? Do you have access to a large telescope?
This fine object is a challenge even through a inch scope. The most famous name related to clusters of galaxies is that of American astronomer George O.
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A supplementary catalog of southern objects contains 1, additional galaxy clusters considered either not rich enough or too distant for inclusion in the main catalog. They are designated by the suffix -s, as in Abell s. NGC looks impressive in this image. Because it measures It would take a huge amateur telescope to reveal the details you see here. Counter reset October Control Center Site Table of Contents. Find an Astronomy Speaker.
AL Observing Programs. PDF File Name.
Messier Club - 70 object and object levels for the telescope. Astronomer's Messier Journal. Binocular Messier Club - 50 of the best Messier objects for binoculars. Deep Sky Binocular Club - 60 additional deep sky objects for binoculars. Herschel Club - deep sky objects for the telescope. We will describe the currently accepted model of the universe in The Big Bang.
For the purposes of this chapter, it is enough to know that the current best estimate for the age of the universe is In that case, if we see an object that emitted its light 6 billion light-years ago, we are seeing it as it was when the universe was almost 8 billion years old.
If we see something that emitted its light 13 billion years ago, we are seeing it as it was when the universe was less than a billion years old. Detailed analysis of such lines can also indicate the types of stars that inhabit a galaxy and whether it contains large amounts of interstellar matter. Unfortunately, many galaxies are so faint that collecting enough light to produce a detailed spectrum is currently impossible.
Astronomers thus have to use a much rougher guide to estimate what kinds of stars inhabit the faintest galaxies—their overall colors. Look again at Figure 1 and notice that some of the galaxies are very blue and others are reddish-orange. Now remember that hot, luminous blue stars are very massive and have lifetimes of only a few million years.
If we see a galaxy where blue colors dominate, we know that it must have many hot, luminous blue stars, and that star formation must have taken place in the few million years before the light left the galaxy. Another important clue to the nature of a galaxy is its shape. Spiral galaxies can be distinguished from elliptical galaxies by shape.
Observations show that spiral galaxies contain young stars and large amounts of interstellar matter, while elliptical galaxies have mostly old stars and very little or no star formation. Elliptical galaxies turned most of their interstellar matter into stars many billions of years ago, while star formation has continued until the present day in spiral galaxies.
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If we can count the number of galaxies of each type during each epoch of the universe, it will help us understand how the pace of star formation changes with time. As we will see later in this chapter, galaxies in the distant universe—that is, young galaxies—look very different from the older galaxies that we see nearby in the present-day universe.
In addition to looking at the most distant galaxies we can find, astronomers look at the oldest stars what we might call the fossil record of our own Galaxy to probe what happened in the early universe. Since stars are the source of nearly all the light emitted by galaxies, we can learn a lot about the evolution of galaxies by studying the stars within them.
What we find is that nearly all galaxies contain at least some very old stars. For example, our own Galaxy contains globular clusters with stars that are at least 13 billion years old, and some may be even older than that. Therefore, if we count the age of the Milky Way as the age of its oldest constituents, the Milky Way must have been born at least 13 billion years ago.
As we will discuss in The Big Bang , astronomers have discovered that the universe is expanding, and have traced the expansion backward in time. In this way, they have discovered that the universe itself is only about Thus, it appears that at least some of the globular-cluster stars in the Milky Way must have formed less than a billion years after the expansion began. Several other observations also establish that star formation in the cosmos began very early.
Astronomers have used spectra to determine the composition of some elliptical galaxies that are so far away that the light we see left them when the universe was only half as old as it is now. Yet these ellipticals contain old red stars, which must have formed billions of years earlier still.