The Moon and the galactic center
Published on Mar 1, 2008 at 9:38 pm.
4 Comments.
Filed under astronomy.
The Moon appears to move its own width across the sky each hour. This morning, the Moon moved from the constellation Scorpius into the Constellation Sagittarius. That isn’t really anything special. It happens about every 27.3 days. However, the Moon does not always follow the same path around the sky. The Moon’s orbit is pretty much along the ecliptic, but it moves a few degrees north or south of the ecliptic.
This morning, between 5 and 6 am here in Texas, the Moon passed right in front of the center of the Milky Way galaxy. I made the image above using Starry Night Pro for 5:35 am. At that time, the Moon would have been in the way of the galactic center.
This is nothing that most people would have ever noticed. The center of the galaxy is locate a long way from here. It is some 26,000 light years away, near the Sagittarius-Scorpius border (just barely on the Sagittarius side of the line between the two). It wasn’t until early in the Twentieth Century that astronomers knew that the center of the galaxy was in that direction. You can’t see it using visible light. The galaxy is filled with dust and gas that limits our views of our own galaxy. There is so much dust and gas that we can only see a few thousand light years within our galaxy. So, for a long time, most astronomers thought that we were near the center of the galaxy, since we could see about as far in any direction. Harlow Shapley, though, plotted positions of globular clusters and found that they were centered at a spot far beyond where most people thought that the galaxy ended. Later, the actual distance to the center was pinned down.
In the early days of radio astronomy, the center of the galaxy was found to be a very powerful radio source. It was labeled Sgr A (meaning the first radio source found in the constellation Sagittarius). Later radio telescopes and radio interferometers showed that Sgr A was an extended source in that direction, but a much smaller part of it seemed to be actually right at the center of the galaxy. We call that Sgr A*. That is universally pronounced Sagittarius A star. Sgr A* seems to be a very compact radio source. Furthermore, it seems to vary in fairly short time periods, indicating that it is quite small. While visual light telescopes have trouble seeing through all the dust and gas in the galaxy, infrared telescopes can do much better. Using such instruments, astronomers have found stars that are moving in very fast orbits around the spot that marks Sgr A*. This tells us that there is something with a mass of nearly three million times the mass of the Sun located right at that spot. We don’t actually see anything there, but we know that the mass is there. Given the small size and huge mass, we believe that Sgr A*, the center of the galaxy, is a supermassive black hole. The radio signals coming from Sgr A* are consistent with an accretion disk and relativistic jets coming from material falling into the supermassive black hole.
Of course, an amateur astronomer would have had a hard time knowing that anything was really different about the motion of the Moon this morning. Nothing can be seen of the galactic center with amateur instruments. Only if someone had a radio telescope aimed at the center of the galaxy would anything at all be detected. In that case, the static from the galactic center would have been quieted for a while as the Moon blocked the signals from reaching the telescope.
-Astroprof
John Hickey on July 23, 2008 at 7:22 pm: 1
Thanks for your site. I had a question. Since I learned that we sight through the ecliptic at Sagittarius to look toward the galactic center, I’m been trying to puzzle out why that is (it seemed at first like a great coincidental cosmic cross), and now I think I get it that the solar system must be rotating under the gravitational pull of the rotating galaxy and that therefore the ecliptic, the plane along which the solar system rotates, is aligned with the rotational plane of the galaxy. Have I got it right, do you think?
Thanks for your time.
John Hickey (human)
West Virginia
John Hickey on July 23, 2008 at 8:07 pm: 2
Me back again. I realized that, if my thinking was right, the ecliptic should parallel the plane of the rotating galaxy - the rotation of the solar system should be in the same plane as the rotation of the galaxy - rather than cross the plane of the galaxy. So I’m still in the dark.
Astroprof on July 23, 2008 at 10:56 pm: 3
John,
The ecliptic is not in the same plane as the galaxy. But, it crossed the plane of the galaxy in two places. One of those places is in Sagittarius. It is not exactly in the same place as the center of the galaxy, but it is close. The Moon’s orbit is far enough from the ecliptic that it can occasionally pass in front of the center of the galaxy. Over time, the ecliptic and the Moon’s orbit shift. So, this is a bit of a coincidence that they cross the plane of the galaxy near the direction of the center of the universe.
John Hickey on July 24, 2008 at 5:14 pm: 4
I understand that the simultaneous alignment of the moon and the ecliptic ia a temporary coincidence. But are you confident that it is a random coincidence that the plane of the ecliptic just happens to cross the plane of the galaxy in such a way that it persistently aims close to the galactic center? That seems quite a coincidence. Is it possible, do you think, that forces we don’t understand might persistently cause such an alignment? Has the plane of the ecliptic been aligned in a different way in the past, do we know?