Astronomy Night: Andromeda

I went to the Monthly Observatory Night at Harvard's CfA tonight. I had missed most of the fall ones due to cooking classes.

Tonight, Pauline Barmby of the CfA talked about the Andromeda galaxy (known hereafter as M31). She works on the team running the Infrared Array Camera on the Spitzer Space Telescope.

M31 is big and fuzzy. The first recorded viewing was in 900AD. It's about the width of 7 full moons in the sky and is better seen with binoculors than a small telescope. It's 180,000 light-years across and is 2.2 million light-years away from us. It's the nearest large spiral galaxy to us.

It was Edwin Hubble looking at M31 in the 1920s from the Mt Wilson Observatory that first saw individual (Cepheid variable) stars in it and and concluded that it was not part of the Milky Way but was in fact a distant galaxy of its own. At the time it wasn't known that there were other galaxies outside of our Milky Way.

Barmby structured her presentation around observations of M31 via 3 of the great observatories available to us: Hubble, Chandra, Spitzer.

The Hubble Space Telescope gives us a view of M31 in visible light. In 1993 we saw a 2nd brighter spot in the center of M31. There are bright blue (young) stars moving fast in the center. For them to be moving so fast there's probably a black hole in there (exerting lots of gravity as they do). But it's a mystery how young stars formed so close to a black hole (they couldn't have formed someplace else and moved there because they are too young).

Hubble also let us see globular clusters in M31. These are "small" collections of 1 million stars orbiting a galaxy. M31's globular clusters are about the age of the globular clusters orbiting our galaxy. Most globular clusters don't have a black hole in the center as many galaxies do, but one of the gc's in M31 does have a black hole. Because of this they think it might be a remnant of a galaxy that was absorbed by M31 and not a globular cluster. A 3.5 day exposure image allows individual stars in the outer halo of M31 to be seen and hense studied (composition, age, etc.) Another mystery is that they seem to be younger than the stars in our galactic halo. Though she always pointed out, that measuring stuff in our galaxy, from within it, is very difficult. It's like an amoeba in the Pacific Ocean trying to see (and measure) all of the ocean from shore to shore.

The Chandra X-Ray Observatory sees not visible light, but x-rays. The Earth's atmosphere blocks x-rays (good for us) so Chandra has to be outside of the atmosphere and it's good to be away from Earth's radition belt. It's orbit is not centered on the earth but next to it and takes it 1/3 of the distance to the moon.

X-Rays are emitted by hot gas, so usually black holes, neutron stars, white dwarfs (things left over when stars die) as well as binary stars (stars that orbit each other and pull material from one to the other). With Chandra we found that the black hole at the center of M31 was too cool, but superimposing visible Hubble pictures they found they were looking at the wrong thing. They found right next to what we thought was the (cool) black hole at the center the real black hole at the center and it was similar temperature to what we expected.

The Spitzer Space Telescope looks at the infrared, which show heat. So, you want it to be very cold so that it's own heat doesn't mess with the observations. So Spitzer is far from earth, in fact it's not in Earth orbit but in orbit around the sun, sorta trailing behind the earth. Also, it's cooled with liquid Helium which will last about 5.5 years. It was lauchned in August of 2003. M31 looks like this from Spritzer:


Combining the heat from these images with the luminisity of Hubble images, lets them estimate that M31 has about about 1 trillion stars, which is a lot for a galaxy. The red stuff you see is warm dust. Astronomers analyze the shape of the rings to figure out what's happening in the galaxy. Notice the bulge at the left of the image, how did that form? There's the other one at the lower right which they think is formed by the gravitational effects of M32 which is the bright spot at about 5 o'clock in the image. There's an inner ring in the center but it's off center from the big ring, how did that form? The guess is that 210,000,000 years ago M32 crashed through the center of M31 and caused these disturbances. There's also a theory that in about 3 billion years M31 will crash into the Milky Way. She had a pretty animation of this future cataclysm that she played a few times :)

She took questions at the end and I asked if they knew what the dust they kept referring to in many of images was and if it was all the same stuff. She said some of it was this multi-word incredibly long technical term which she then said the black stuff you scrap off your BBQ is the same stuff. She said that if her study gets chosen for telescope time she might be able to answer what the other stuff was. :)

It was another fun evening, even if it was too cloudy to do any observing on the roof afterwards.