All posts for the month May, 2008

The Milky Way Loses Mass the Easy Way

So, let’s say you’re an average spiral galaxy doing your thing in the Local Group. You have a hundred billion stars (give or take), some nebulae, a big core (with a black hole or two hidden in the center), and some planets (but you’re not sure how many). But, according to astronomers who have measured you, it seems that you’re much more massive than you thought. Could it be all that dark matter you’ve been carrying around with you since you were a young galaxy? What’s the deal here?

So, you do like everybody else who’s overweight does — you think about going on a diet. Galactically speaking, the only way you’re going to do that is lose mass, which is tough to do. In fact, short of having a near-miss with another galaxy (which would tear away some of your stars and mass), it’s not easy for a middle-aged galaxy to lose weight.

But wait! There’s another way. Maybe, you think, they didn’t weight you correctly. Yeah, that’s the ticket. The scale is off a little…

As a matter of fact, that’s exactly the issue that scientists set out to solve, using the Sloan Digital Sky Survey to measure stars in the galaxy and recalculate its mass based. Basically, they did a more efficient survey which allowed them to come up with a better estimate of the true mass of our galaxy. The difference between the older, fatter Milky Way and the new, improved, skinnier one is a pile of mass equivalent to a trillion Suns.

“The galaxy is slimmer than we thought,” said Xiangxiang Xue of the National Astronomical Observatories of China, who led an international team of researchers on the project. “That means it has less dark matter than previously believed, but also that it was more efficient in converting its original supply of hydrogen and helium into stars.”

Our sun lies about 25,000 light years from the center of the galaxy, roughly halfway out in the galactic disk. The new mass determination is based on the measured motions of 2,400 “blue horizontal branch” stars in the extended stellar halo that surrounds the disk. These measurements reach distances of nearly 200,000 light years from the galactic center, roughly the edge of the region illustrated above.
The visible, stellar part of our Milky Way in the middle is embedded into its much more massive and more extended dark matter halo, indicated in dim red. The ‘blue horizontal branch stars’ that were found and measured in the SDSS-II study, are orbiting our Milky Way at large distances.

From their speeds the researchers were able to estimate much better the mass of the Milky Way’s dark matter halo, and found it to be much “slimmer” than thought before.

How They Did It

The discovery that the Milky Way is slimmer than we thought is based on data from SEGUE, an enormous survey of stars in the Milky Way — one of the three programs that comprise SDSS-II. Using SEGUE measurements of stellar velocities in the outer Milky Way, a region known as the stellar halo, the researchers determined the mass of the galaxy by inferring the amount of gravity required to keep the stars in orbit. Some of that gravity comes from the Milky Way stars themselves, but most of it comes from an extended distribution of invisible dark matter, whose nature is still not fully understood.

To trace the mass distribution of the Galaxy, the SEGUE team used a carefully constructed sample of 2,400 “blue horizontal branch” stars whose distances can be determined from their measured brightness. Blue horizontal branch stars can be seen to large distances, enabling the team to measure velocities of stars all the way out to distances of 180,000 light years from the sun.

The most recent previous studies of the mass of the Milky Way used mixed samples of 50 to 500 objects. They gave implied masses up to two trillion times the mass of the Sun for the total mass of the Galaxy. By contrast, when the SDSS-II measurement within 180,000 light years is corrected to a total mass measurement, it yields a value slightly under one trilliion times the mass of the Sun.

And that’s how the Milky Way got a little slimmer, without the galactic equivalent of a diet. Wish it were so easy for humans!!

Watch and Listen As Phoenix Descended to Mars

The folks at the European Space Agency have posted a video and sound files made from data recorded on board Mars Express as the Phoenix lander made its way to the Martian surface.

The animation shows the signal of Phoenix’s descent. The spike in the animation, between frequencies of 7 and 8 kiloHertz, shows the transmission from Phoenix itself. The lander’s signal can be seen in the animation starting from about 342 seconds after the start time and disappears at about 1085 s. This shows Mars Express picking up on the Phoenix signal and tracking it while closing in on the lander; the closest Mars Express got to Phoenix was 1,550 kilometers.

As Mars Express flew away, the lander deployed its parachute, separated from it and landed, the signal from the lander was cut off. The shift of the spike seen in the animation, is due to the so-called Doppler effect, which is very similar to what we hear when listening to the whistle of a passing train.
You can hear a sound file of the descent and landing on the same page linked above (warning: if you have tinnitus, turn your speakers down to a lower volume).  The image below is from the Mars Reconnaissance HiRise imager.

Sure, Why Not?

When I was watching the Mars Phoenix lander festivities the other night, I thinking about all the scientists and their students who are (as we speak) working with the incredible rush of data being returned by the mission. And, it occurred to me that this kind of science is something I wish everybody could experience once in their lives. It’s a heady feeling, looking at images and data and realizing that you’re finding something new and interesting to share with the rest of the world.

Amazingly enough, discovery in the universe is NOT limited to scientists, although they’re the ones best trained to undertake the years of work that it takes. But, as I learned in my days on the Halley Watch project, there are a lot of amateurs out there who are also well-equipped (both mentally and with access to equipment) to discover unique things in the universe. Much of the work I did on the Halley Watch project (which culminated in an atlas of Halley images that we used to study the solar wind’s influence on comet plasma tails) came from amateur astronomers who submitted images for study. And they were first-rate images, exactly what we needed.

Today, I got a story about an amateur astronomer named Richard Miles, who used a telescope in Australia that is part of the Faulkes Telescope Network to look at an asteroid called 2008 HJ. His work, conducted via the Internet from his home in Dorset, England, proved that this newly discovered asteroid is rotating (spinning around an axis) once every 42.7 seconds. That makes this object the fastest-known rotator in the solar system. In asteroid studies, this is a big deal, since these little worldlets and chunks of solar system debris are hard to see, let alone figure out how fast they’re rotating!

I am constantly amazed at what there is yet to learn in the universe. What this find tells me is that there’s plenty of discovery in the universe, and it’s not all limited to folks in the big labs. There are an increasing number of robotically controlled telescopes available to interested and well-prepared amateurs who want to do some research. As we used to tell the participants in the Halley Watch, there’s room for everybody in the cosmic pool — from first-time stargazer to well-equipped amateur to professionally trained researcher. Jump on in and take a swim!