Skip to main content

The Milky Way's Mass is Much Lower Than We Thought

How massive is the Milky Way? It’s an easy question to ask, but a difficult one to answer. Imagine a single cell in your body trying to determine your total mass, and you get an idea of how difficult it can be. Despite the challenges, a new study has calculated an accurate mass of our galaxy, and it’s smaller than we thought.

One way to determine a galaxy’s mass is by looking at what’s known as its rotation curve. Measure the speed of stars in a galaxy versus their distance from the galactic center. The speed at which a star orbits is proportional to the amount of mass within its orbit, so from a galaxy’s rotation curve you can map the function of mass per radius and get a good idea of its total mass. We’ve measured the rotation curves for several nearby galaxies such as Andromeda, so we know the masses of many galaxies quite accurately.

But since we are in the Milky Way itself, we don’t have a great view of stars throughout the galaxy. Toward the center of the galaxy, there is so much gas and dust we can’t even see stars on the far side. So instead we measure the rotation curve using neutral hydrogen, which emits faint light with a wavelength of about 21 centimeters. This isn’t as accurate as stellar measurements, but it has given us a rough idea of our galaxy’s mass. We’ve also looked at the motions of the globular clusters that orbit in the halo of the Milky Way. From these observations, our best estimate of the mass of the Milky Way is about a trillion solar masses, give or take.

The distribution of stars seen by the Gaia surveys. Credit: Data: ESA/Gaia/DPAC, A. Khalatyan(AIP) & StarHorse team; Galaxy map: NASA/JPL-Caltech/R. Hurt

This new study is based on the third data release of the Gaia spacecraft. It contains the positions of more than 1.8 billion stars and the motions of more than 1.5 billion stars. While this is only a fraction of the estimated 100-400 billion stars in our galaxy, it is a large enough number to calculate an accurate rotation curve. Which is exactly what the team did. Their resulting rotation curve is so precise, that the team could identify what’s known as the Keplerian decline. This is the outer region of the Milky Way where stellar speeds start to drop off roughly in accordance with Kepler’s laws since almost all of the galaxy’s mass is closer to the galactic center.

The Keplerian decline allows the team to place a clear upper limit on the mass of the Milky Way. What they found was surprising. The best fit to their data placed the mass at about 200 billion solar masses, which is a fifth of previous estimates. The absolute upper mass limit for the Milky Way is 540 billion, meaning that the Milky Way is at least half as massive as we thought. Given the amount of known regular matter in the galaxy, this means the Milky Way has significantly less dark matter than we thought.

Reference: Jiao, Yongjun, et al. “Detection of the Keplerian decline in the Milky Way rotation curve.” arXiv preprint arXiv:2309.00048 (2023).

The post The Milky Way's Mass is Much Lower Than We Thought appeared first on Universe Today.



from Universe Today https://ift.tt/3PKspGt
via IFTTT
Labels:

Comments

Post a Comment

Popular posts from this blog

Researchers Match Up 12 Meteorites with the Near-Earth Asteroids They Came From

Every day meteoroids blast through our planet’s atmosphere to hit the ground as meteorites. A team of researchers in Italy traced twelve of them to progenitor asteroids that orbit in near-Earth space. Scientists treasure meteorites because they reveal information about their parent bodies. In an arXiv paper, two Italian researchers—Albino Carbognani and Marco Fenucci—analyze the characteristics of the parent bodies of 20 selected meteorites. They were able to track all but eight back to their parent asteroids. Based on their work, the pair says at least a quarter of meteorites come from collisions that happened in near-Earth space and not in the Main Belt. Meteorites from Near-Earth Asteroids: How They Got Here Many meteorites are chondritic, similar to asteroids in the Main Belt (or came from it). In their paper, the authors point out that progenitor meteoroids (including many that fall to Earth and become meteorites) formed millions of years ago following collisions between main-...
Post a Comment
Read more

JWST Takes a Detailed Look at Jupiter’s Moon Ganymede

Nature doesn’t conform to our ideas of neatly-contained categories. Many things in nature blur the lines we try to draw around them. That’s true of Jupiter’s moon Ganymede, the largest moon in the Solar System. The JWST took a closer look at Ganymede, the moon that’s kind of like a planet, to understand its surface better. Ganymede is basically a planet, except it doesn’t orbit the Sun. If it did orbit the Sun instead of Jupiter, it would be indistinguishable from a planet. It has a differentiated internal structure with a molten core that produces a magnetic field. It has a silicon mantle much like Earth’s, and has a complex icy crust with a deep ocean submerged beneath it. It has an atmosphere, though it’s thin. It’s also larger than Mercury, and almost as large as Mars. According to the authors of a new study, it’s an archetype of a water world. But even with all this knowledge of the huge moon, there are details yet to be revealed. This is especially true of its complex surface...
Post a Comment
Read more

The Ultraviolet Habitable Zone Sets a Time Limit on the Formation of Life

The field of extrasolar planet studies has grown exponentially in the past twenty years. Thanks to missions like Kepler, the Transiting Exoplanet Survey Satellite (TESS), and other dedicated observatories, astronomers have confirmed 5,690 exoplanets in 4,243 star systems . With so many planets and systems available for study, scientists have been forced to reconsider many previously-held notions about planet formation and evolution and what conditions are necessary for life. In the latter case, scientists have been rethinking the concept of the Circumsolar Habitable Zone (CHZ). By definition, a CHZ is the region around a star where an orbiting planet would be warm enough to maintain liquid water on its surface. As stars evolve with time, their radiance and heat will increase or decrease depending on their mass , altering the boundaries of the CHZ. In a recent study , a team of astronomers from the Italian National Institute of Astrophysics (INAF) considered how the evolution of star...
Post a Comment
Read more