6degrees astroblog with Irwin Horowitz

A Universe Filled With Galaxies

By Irwin Horowitz, 4-25-07

Well, it looks like we’ve got ourselves a new astronomy blog here at NewWest.Net/Boise. I hope that in the months to come, that this proves to be an enjoyable column for me to write and for you to read.  My main columns will focus on some astronomical topic relevant for that month, along with some insights and discussion of what is currently visible in the night time sky.  On occasion, when a news story breaks of astronomical interest, I will post a column about that topic on the blog.  I would also like to utilize this column as a resource for astronomy clubs throughout the western third of the country (Rocky Mountains and Pacific coast) to advertise their events like monthly meetings, star parties and community activities.  Lastly, if my readers would like me to post columns of educational value on a topic of interest, for example, black holes, I will certainly do my best to accommodate such requests.

Now, let us turn our gaze from matters terrestrial upwards towards the heavens…

Last month, in a guest post on New West.net, I related the story of French comet hunter Charles Messier, his catalog of objects that he would confuse with comets and how present day amateur astronomers host “Messier Marathons” each year in the early spring to view these objects.  On a personal note, here in Idaho we had a wonderful time at Bruneau Dunes State Park over Messier Marathon weekend, and on Friday night, I managed to observe 91 of the 110 objects in the catalog!  This month I want to turn our focus to objects a little further away than comets…galaxies!

Our solar system, the Sun, the eight planets (remember Pluto’s been officially demoted), and the various small bodies are located within the disk of our home galaxy, the Milky Way.  The Milky Way is a barred spiral galaxy.  The vast majority of the stars, gas and dust in our galaxy are located in a disk shape whose nature can be clearly observed on dark summer nights.  We are orbiting about the center of this disk, about 2/3 of the way out to the visible edge.  Our galaxy is so large that it takes us nearly 250 million years to make a single orbit about the center, resulting in about 18-20 orbits since the formation of our solar system 4.5 billion years ago.

Because of the shape of the galaxy, there are directions in the sky where we are looking perpendicular to the plane of this disk, and these directions are most readily observed during the spring and autumn seasons.  When looking out of the plane of the disk, the gas and dust that would block out the light from more distant objects are much less prevalent.  This permits us to observe these distant objects beyond our galaxy.  As a result, at this time of the year, in the northern hemisphere, we are fortunate to be able to observe two huge clusters of galaxies far beyond the Milky Way.  One is located in the constellation of Virgo at a distance of 60 million light years and consists of more than a thousand galaxies.  Many of the galaxies in this cluster were noted by Messier in his catalog, including a monstrous elliptical galaxy in its heart, M87.  This galaxy exhibits strong radio emission and there is a powerful jet of material shooting out from its core.  It is conjectured that this core contains a supermassive black hole, with a mass greater than 1 billion times the mass of our own Sun.

The second cluster of galaxies appears in the sky near the Virgo cluster in the constellation of Coma Berenices, but the members of the Coma cluster are more than five times further away than the Virgo cluster.  As such, these galaxies were too faint to be seen by Messier and none of them are in his catalog.  This cluster is so far away that we can use measurements of the motions of its members to get an accurate value for the rate at which our universe is expanding.  According to the observations made by Edwin Hubble in the 1920s and 1930s, galaxies which are further away from us are moving away at a greater velocity.  A cluster like Virgo is so close to us that a significant portion of the measured motions of its constituent galaxies is due to effects other than the expansion of the universe.  For the Coma cluster, since it is so much further away, its members are moving away from us at a much greater velocity, and therefore most of the motion of its galaxies is the result of the expansion.

As an analogy to explain this effect, suppose there was a bus full of people driving away from you and the passengers were all walking around randomly inside the bus.  At the same time there is an airplane in the sky, much further away, and flying away from you much faster than the bus.  The passengers on the plane are moving around much like those on the bus.  You want to determine the velocities of both the bus and the plane by measuring the velocities of the passengers.  Onboard the bus, the individual motions are each a sizable fraction of the velocity of the bus, while on the plane, their motions are only a tiny fraction of the overall velocity of the plane.  In this analogy, the passengers represent the galaxies in each cluster, the bus is the nearby Virgo cluster and the plane is the more distant Coma cluster.

Over the past century, astronomers have observed thousands of galaxy clusters like the two mentioned above.  It is indeed believed that there are as many galaxies in our universe as there are stars in the Milky Way (several hundred billion).  This means that there are perhaps several sextillion (a billion trillion) stars in our universe.  One can only conjecture about what sort of worlds might circle many of those stars, and what lifeforms might have evolved on them over the 13 billion year lifetime of our universe.
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As winter turns to spring, the weather begins to become more amenable to spending time outdoors after sunset.  Going outside, you’ll continue to see Venus dominating the western sky, which it will do until July when it passes between the Earth and the Sun.  Saturn will also be prominently displayed in Leo, to the west of the sickle shape that is the lion’s mane, and whose base is the bright star Regulus,.  Saturn, of course, is the crown jewel of our solar system, and the view through even the most modest of telescopes is phenomenal.  The rings are clearly observed encircling the planet, and when viewed through a more powerful instrument, it is possible to see some of the structure in the rings like the Cassini division.  Last month, during the Messier Marathon here in Idaho, the view of Saturn was particular awe inspiring, as the image was sharp and crisp even under high magnification. 

You can also see some of the brighter moons orbiting the planet, in particular the moon Titan.  A few years ago, a NASA spacecraft named in honor of the Italian astronomer Cassini, who first observed the rings, arrived at the planet.  One of the parts of that mission consisted of the Huygens probe which detached from the main spacecraft and dropped into the atmosphere of Titan, landing on its surface and relaying data back home of the conditions it encountered.  What it found is that the surface temperature is almost 300 degrees below zero Fahrenheit!  There are lakes consisting of liquid methane and rain falling from clouds of organic molecules.  Vastly different than what we have here on Earth. 

Kicking off my list of Astro-events for May are two activities with the Boise Astronomical Society.  On Friday, May 11, the BAS will hold its monthly membership meeting in Classroom B of the Discovery Center of Idaho (DCI) at 7 p.m.  We will have a presentation by Chris Anderson on the Hubble Space Telescope.  On Saturday, May 19, the BAS joins the Bogus Basin Foundation for our annual public star party in the parking lot near the ski lodge.  The event kicks off at 7 p.m. with observing the Sun before it sets.  After dark, we will be treated to a crescent moon next to Venus as well as magnificent Saturn.  Towards the end of the evening, mighty Jupiter will come into view as a conclusion to the festivities.  Until next time, turn off your lights and turn on to astronomy!

Publisher’s note: Irwin Horowitz is a real rocket scientist who lives in Boise. He has six college degrees, and is pretty smart.

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