Just in Time for Summer: The Milky Way Loses Weight

Article written: 18 Jun , 2008
Updated: 24 Dec , 2015

Have you ever been surprised at your annual weigh-in at the doctor’s office to find that your bathroom scale at home was wrong? Or, bought a new scale that had a difference of opinion with your old one? That’s what has happened with our very own Galaxy, the Milky Way. “The Galaxy is slimmer than we thought,” said Xiangxiang Xue of the Max Planck Institute for Astronomy in Germany and the National Astronomical Observatories of China, who lead a research team using the Sloan Digital Survey to measure the mass of the stars in the galaxy. “We were quite surprised by this result,” said Donald Schneider, a member of the research team, from Penn State. The researchers explained that it wasn’t a Galactic diet that accounted for the galaxy’s recent slimming, but a more accurate scale.

The researchers used the motions of distant stars to make the new determination of the Milky Way’s mass. They measured the motions of 2,400 “blue horizontal branch” stars in the extended stellar halo that surrounds the disk of the galaxy. These measurements reach distances of nearly 200,000 light years from the Galactic center, roughly the edge of the region illustrated in the image above. Our Sun lies about 25,000 light years from the center of the Galaxy, roughly halfway out in the Galactic disk. From the speeds of these stars, the researchers were able to estimate much better the mass of the Milky Way’s dark-matter halo, which they found to be much ‘slimmer’ than thought before.

The discovery is based on data from the project known as SEGUE (Sloan Extension for Galactic Understanding and Exploration), an enormous survey of stars in the Milky Way. 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 the distribution of invisible dark matter, which is still not fully understood.

The most recent previous studies of the mass of the Milky Way used mixed samples of 50 to 500 objects. They 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-trillion times the mass of the Sun.

“The enormous size of SEGUE gives us a huge statistical advantage,” said Hans-Walter Rix, director of the Max Planck Institute for Astronomy. “We can select a uniform set of tracers, and the large sample of stars allows us to calibrate our method against realistic computer simulations of the Galaxy.” Another collaborator, Timothy Beers of Michigan State University, explained, “The total mass of the Galaxy is hard to measure because we’re stuck in the middle of it. But it is the single most fundamental number we have to know if we want to understand how the Milky Way formed or to compare it to distant galaxies that we see from the outside.”

All SDSS-II observations are made from the 2.5-meter telescope at Apache Point Observatory in New Mexico. The telescope uses a mosaic digital camera to image large areas of sky and spectrographs fed by 640 optical fibers to measure light from individual stars, galaxies, and quasars. SEGUE’s stellar spectra turn flat sky maps into multi-dimensional views of the Milky Way, Beers said, by providing distances, velocities, and chemical compositions of hundreds of thousands of stars.

Source: Penn State, arXiv

9 Responses

  1. Nephish777 says

    Too bad I can’t loose weight like that!

  2. Al Hall says

    Well, you could just cut away some of that invisible dark matter.. 🙂

  3. Dan says

    The paper in its conclusions actually says that the new value sits smack in the middle of previous – but current – values: no “weight loss” evident here …

  4. Dan says

    Isn’t that interesting: Universe Today only accepts my comments when I use an invalid e-mail address but not the real one. What’s going on here …

  5. Adérito Luís says

    Hello I´m a new one and want to know the difinitio of dark-matter. Dark-matter, what´s that´s that?

  6. Patrick says

    Milky Way Georgraphy —
    OK – We (the Sun) is about 25,000 light years out from the center of the galaxy –
    Does anyone know the angle between the plane of the solar system (the ecliptic) and the plane of the Milky Way disk (the galactic plane) — ????
    Does it change with our rotation about the galactic center ? If so how does it change, and who’s keeping records on this?

  7. ray says

    Adérito Luís, I hope this helps.
    Dark matter is the matter that doesn’t directly reflect or emit enough electromagnetic radiation to be visable. Only about 4 percent of the matter in the universe is seen and about 24 percent is dark matter. It includes black holes that don’t allow the escape of light along with dwarf stars and planatoid objects that don’t emit or reflect enough light to be seen. The rest of the universe is dark energy which is commonly confused with dark matter. It is the largest chunk of the universe at nearly three quarters of all matter. Dark matter may include neutrinos and other elementary particles like axions and WIMPs. Dark energy is much harder to define, but it seems to exist and has a gravitaional effect on other matter. Gravity may be the only way in which it reacts with matter as we know it. It produces pressure and causes gravitaional repulsion. It may or may not be homogeneous in nature and it certainly gives rise to science fiction stories. It is calculated by its effect on matter and not anything measured directly. Matter as we know it, is made of protons and electrons and defined by the number of protons in the nucleus. A proton is a proton regardless of what element it is found in but who is to say that all matter in the universe uses the same basic building blocks?

  8. Al Hall says


    ray –
    Forgot to mention that dark matter (and dark energy) is theory. Not yet fact.
    I’m a firm believer that EVERYTHING is made of something. I don’t believe in “nothing” as we perceive, at least. When we look at “empty” space, I believe it is made of “something”.. and I guess that the building blocks – as we know of- are atoms-.. So, yes.. There could be trillions of cubic kilometers of space (filled with trillions and trillions of atoms) surrounding galaxies and connecting everything, thus could possibly have a gravitational effect.. But for the moment it is still theory.. I hope that we will know the answer soon.

  9. Alphonso Richardson says

    Looks like they didn’t include all that buggering about with dark matter.
    Thank goodness.

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