Beyond wrote:Ann is going to love the "end of the river" in this APOD!!
But I also love the difference in color between our own galaxy and the Magellanic Clouds. The two Clouds of Magellan
are really dominated by young blue stars in a way that make them so different from our own galaxy.
In his book The Galaxies of the Local Group
, Sidney van den Bergh discussed the star formation of the Magellanic Clouds. His discussion of the Large Cloud is the most interesting. Sidney van den Bergh wrote:
The idea that a violent burst of star formation in the LMC followed a rather lengthy period of quiescence was first introduced by Butcher(1977).
Butcher concluded that a dramatic increase in the rate of star formation had taken place in the LMC 3-5 Gyr ago.
Bertelli et al. (1992) estimated that the mean rate of star formation in the Large Cloud was as much as ten times higher after the burst had started than it had been previously.
From their observation of a field located (..) from the center of the LMC Elson et al. (1997) found that only ~5% of the stars in this field belong to the old disk population that predated the great burst of star formation that started ~3 Gyr ago.
Teh Large Magellanic Cloud. Photo: Alan Dyer.
The expression "Gyr" means "billion years. The gist of what Sidney van den Bergh wrote here is that most of the stars in the Large Magellanic Cloud seem to have formed in the last 3 billion years. Fascinatingly, van den Bergh speculated that the Large Magellanic Cloud might have been a mild example of a low surface brightness galaxy
before it suddenly ran into starburst mode about three billion years ago. And while the greatest burst of star formation in the LMC took place about 3 billion years ago, the LMC keeps forming new stars at a very high rate.
This is so different from our own galaxy, which probably formed most of its stars about 10 to 12 billion years ago or so, and its rate of star formation has likely been decreasing ever since then. Because our own galaxy contains so many old stars, it has a large yellow bulge, an old yellow thick disk and huge amounts of dust from eons of star formation. The Magellanic Clouds contain no bulges and little dust.
Other large galaxies typically have satellite galaxies
that are mostly devoid of star formation. What makes the Magellanic Clouds so different? Although I can't find the source now, I know there has been speculation that the Magellanic Clouds might have only rather recently been captured by the Milky Way. It is fascinating to speculate that the great burst of star formation in the Large Magellanic Cloud (and to a lesser extent in the Small Magellanic Cloud) might have something to do with them being captured by our own big bully of a galaxy and having their gas stirred up and jostled by the powerful gravity of the Milky Way.