Date: March 19, 2012
Title: Astronomy Word of the Week : Globular
Podcasters: Dr. Christopher Crockett
Organization: United States Naval Observatory
Links: http://www.usno.navy.mil/USNO
http://astrowow.wordpress.com/
Description: Come learn about some of the oldest relics in our Milky Way Galaxy and how they helped forever change humanity’s view of its place in the Universe. The astronomy word of the week is “Globular”!
Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Sponsor: “This episode of 365 days of Astronomy is sponsored in-kind by The Planetary Society.”
Transcript:
Right smack in the middle of the southern constellation Centaurus (a centaur) is a relatively bright star – easily visible to the naked eye – that goes by the unassuming name omega Centauri. As stars go, it’s not terribly bright and not terribly faint. To the causal observer, you probably wouldn’t give it a second glance. But turn a telescope on to it and this “star” becomes something else entirely. Edmund Halley – best known for his eponymous comet – identified it as a nebula rather than a star because it took on a distinctly “fuzzy” appearance upon closer examination. It wasn’t until 1830 when John Herschel – who’s father discovered the planet Uranus roughly 50 years earlier – revealed the true nature of omega Centauri. It is not a star, nor a nebula, but a massive conglomeration of stars known to astronomers as a globular cluster.
Star clusters are broken in to two major catagories: open and globular. Open clusters are loose collections of anywhere from dozens to thousands of relatively young stars. Globulars are something else entirely. They are dense stellar fossils from the earliest epochs of our Galaxy’s history. They are essentially enormous balls of stars – the term globular in fact comes from the latin word for sphere – each containing hundreds of thousands of stars. These spheres are typically a hundred or so light-years across and are scattered throughout a tenuous halo that surrounds the Milky Way. The stars in these clusters are old – very, very old. Globular clusters are, in fact, among the oldest objects in our Galaxy, relics from the dawn of the Universe. How they got here and what they can tell us about the history of the Milky Way and the formation of other galaxies still remains very much a mystery.
The first globular was identified in 1665 by an amateur German astronomer – a cluster which would come to be known as M22. The brightest globular visible to the naked eye in the northern hemisphere, its intricate structure remained hidden to the modest telescopes of the day. All that could be said was that it was definitely not a single star, and was referred to simply as a round nebula. Halley’s identification of omega Centauri made that cluster the second to be identified. The details of globulars would have to wait until 1764 when comet hunter Charles Messier was able to see individual stars in the cluster known as M4. Today we not only know of roughly 150 globulars in our own Galaxy, but we also know about the globular population in other galaxies as well. The massive galaxy M87 sitting at the core of the Virgo Galactic Cluster harbors somewhere in the neighborhood of 10,000 such clusters!
While we know of many globulars residing in the Milky Way, they almost seem to live a life somewhat apart from the rest of the Galaxy. Our Milky Way is shaped like a thin disk with a slight bulge in the center. The roughly 100 billion stars in the disk all orbit about the center in roughly the same direction and same plane. Globulars do their own thing, however. They orbit about the Galactic Center in a gigantic sphere extending beyond the edge of the primary disk – the Galactic Halo. Their orbits take them high above the Galactic plane and then plunging through the spiral disk, possibly shedding stars every time they do so, only to fall far below, turn around, and do it again.
In the early part of the last century, American astronomer Harlow Shapley noticed that globulars were distributed about the sky rather oddly. If you were to go out and count up the number of stars in a given patch of sky, you’d find that the stars are more or less evenly distributed about the Earth. With the exception of the band of the Milky Way, any one patch doesn’t really have more or fewer stars than any other patch. For a long time, this was used to support the notion that our Solar System sat roughly in the center of the Galaxy (or actually, the Universe – no one yet realized that some of the distant nebula they were seeing through their telescopes were actually entire galaxies!). Shapley reported, in 1918, that globulars tended to favor one side of the sky over the other. The summer sky had more globular clusters than the winter sky. To him, this suggested that these clusters were actually orbiting around something very massive but as yet unseen. That the globular clusters were congregating around somewhere else hinted that the center of the Galaxy was actually quite a ways off from Earth.
Shapley actually tried to estimate how far we sat from the center of the Galaxy. Though he was pretty far off in his numbers (mostly because he didn’t realize that interstellar dust was making the globulars look dimmer than they otherwise would appear), he definitely had the right idea. The globulars provided the first hints of the true structure of our Galaxy and of our place in it. The globular clusters are orbiting around the center of our Galaxy – some 28,000 light-years from our home.
But what really distinguishes globular clusters from the rest of the Galaxy is their immense age. Most are approximately 13 billion years old, just a billion or so years younger than the Universe itself. They are no longer forming new stars – they exhausted their supply of raw hydrogen long, long ago. Their age gives us a peek into the processes at work in the early Universe. Because they are massive populations of stars that all formed out of the same material and at roughly the same time, they are a unique laboratory for unraveling the secrets of the stellar lifecycle – the birth, life, and eventual death of stars. Despite this rare glimpse they afford us into the workings of the Cosmos, they still remain very much an enigma. How they formed, where they came from – these continue to be unanswered questions. It is thought that they may be remnants of our Galaxy’s cannabilistic past where the Milky Way grew by consuming smaller galaxies. Globular clusters may therefore be the remnant cores of these ancient victims of our Galaxy’s relentless gravitational pull. But, like much in astronomy, nothing is certain.
End of podcast:
365 Days of Astronomy
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