September 23, 2019

How Far Can YOU See?

How far can you see with that thing?” I hear that one a lot at star parties. It’s a sort of ambiguous question to an astronomer; distance varies with the scale of the structures in our universe observed, and a myriad of factors both near and far conspire to make this number an enigma not as straight forward as it might sound. On the Earth, we’re limited by the atmosphere and the curvature of the planet to a line of sight of maybe 50 or so miles on a clear day from a high mountain top… but in space…

This week, as nights are moonless and relatively dark, I’m going to introduce you to the most distant object you are likely to see with the naked eye from suburban backyard skies; the Andromeda galaxy, in the constellation of the same name, about 2,500,000 light years away. One of the nearest large galaxies and the best studied, it has been noted by observers as a fuzzy patch since 905 A.D. When recorded by the Persian astronomer Al Sufi. The magnitude of this object, also known as M31 or NGC 224, is usually given as +5.0, but keep in mind that this is spread over a huge surface area of 160′ x 40′, or the visual area of about 10 full moons! Of course, that means the actual surface brightness of the galaxy suffers greatly from any existent light pollution, and what you’re more than likely to see with binoculars or a small telescope is the relatively bright nucleus of the galaxy. To find the Andromeda galaxy, start from the great “box” of Pegasus, (see the finder chart above) which currently rides high in the east in the late Fall evenings. From the upper (“up” being “northern”) left corner star, trace out the horn shaped outline of the constellation Andromeda. (OK, we can’t see a legendary maiden, either!) Just above the widest part of the horn is the star Nu Andromedae, and sweeping 1 degree west of the star at low power will easily sweep up the huge galaxy. Two other fuzzies may be readily apparent in the same field, which are the related satellite galaxies M110 and M32. Although you won’t see all of them, the Andromeda galaxy has 14 known satellite companions. Andromeda has about an estimated one trillion stars, and is more than twice as massive as our own Milky Way. The Andromeda galaxy is also bound to our own local group of galaxies, and we’re approaching each other at 190 miles a second, for an eventual collision in about 2.5 billion years. Of course, most of space even within galaxies is nothing, so its more than likely that we’ll both become distorted and either reabsorbed, or merge into one huge mega-galaxy… least likely but intriguing is the concept that our solar system may actually get ejected from the galaxy entirely… what a view we would have then!

The astro word of the week is Cepheid variable. This is a particular type of variable star that has a luminosity that is in direct proportion to its pulsation period, allowing its use as a standard candle over extragalactic distances. The first Cepheid discovered was the star Delta Cephei in the northern constellation Cepheus, hence the name. Another famous Cepheid is the north pole star Polaris, which is also the nearest Cepheid at about 430 light years distant. Distance to the nearer Cepheids is nailed down by stellar parallax. Discoveries of Cepheids in the Andromeda galaxy by Edward Hubble in the 1920′s gave the first clues that these wispy patches were truly island universes of their own. These were in the order of 10 magnitudes fainter than Cepheids found in our galaxy, hence the Andromeda had to be at an immense distance, since light, like sound, varies intrinsically with distance (twice as far is ¼ as bright, 4 times is 1/16, etc…) Its hard to imagine that less than a century ago, the universe was thought to span only the extent of our Milky Way!

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