Category Archives: Milky Way Galaxy

astronomy news, astrophysics, galaxies, Milky Way Galaxy

Gaining Clarity on the Milky Way

Leave a comment

Learning More about the Home Galaxy

Wow. I miss ONE AAS meeting, and they lose two spiral arms from our galaxy! This is what I get for staying home and working on two big projects with massive deadlines. I did manage to log in and watch the press conferences from the comfort of my overstuffed office, so I caught the gist of what has turned out to be a very cool story.

First, some background. For years, astronomers thought our galaxy probably had four spiral arms. But, it’s hard to tell because we can’t exactly SEE all of our galaxy in visible wavelengths. This is due to all the dust in the galactic plane, and since WE’RE in the galactic plane, it’s like trying to see through a dense fog when it comes to looking in various directions. Isaac Asimov put it best when he wrote, “In a sense, we are on a low roof on the outskirts of the city on a foggy day.”

Well, as it turns out, telescopes with infrared capability can look through that fog and see farther and sharper than visible-light counterparts. So, it makes sense to use such facilities when you want to see what’s out there.

Groups of astronomers from JPL/CalTech and the University of Wisconsin used the infrared-enabled Spitzer Space Telescope to look at the galaxy. Their survey, called the Galactic Legacy Infrared Midplane Extraordinaire (GLIMPSE), spanned 130 degrees in longitude (65 degrees on either side of the center of the galaxy), and 2-4 degrees in latitude. Since Earth is located about halfway out along the plane of a flattened spiral, this survey actually encompasses a large fraction of the volume of the Milky Way. The infrared views (shown below) show where the stars are, as well as the dust clouds in the plane, allowing a more complete star census to be done.

[GLIMPSE]

Here’s one of two views of the Milky Way from GLIMPSE, emphasizing wavelengths (3.6 – 8 microns) in the familiar blue-green-red that our eyes see, with the shortest wavelengths displayed in blue and the longest in red. For more technical details, check out the GLIMPSE page at the University of Wisconsin’s team site. They also have downloadable poster-size files of this and another view of the same data.

Milky Way Spiral StructureNow, if you could turn that image so we could see it from the “top down” this “artist’s concept” might be what our Milky Way really looks like. The survey showed, for the first time, that arms we thought were there, really aren’t. The Milky Way’s elegant spiral structure is dominated by just two arms wrapping off the ends of a central bar of stars.

Spitzer’s imagery of the galaxy comprises 800,000 snapshots, and when you piece them all together, they make the clearest view of the Milky Way ever produced. The team has created a poster that they showed at AAS that is 180 feet (289 meters) long and 3.5 feet (2.1 meters) wide. The poster is going on tour soon, apparently showing up at Griffith Observatory (where they have a similar large-scale image of a small part of the Virgo Cluster called “The Big Picture” on display), and then on to various other cities. You, however, can explore it from the comfort and privacy of your home or office by going to http://mipsgal.ipac.caltech.edu/p_map.html where they have a GoogleMaps zoomable map. Check it out!

astronomy, astronomy news, galaxies, Milky Way Galaxy, updates and news

Does This Spiral Arm Make Me Look Fat?

Leave a comment

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!!