How do you weigh a galaxy? It’s an astronomical challenge, particularly if it’s the galaxy you call home. It turns out there are several ways to get a handle on the mass of the Milky Way, and a recent study summarizes these methods to present the best value.
One method is to look at the motion of stars in the galaxy. Most Milky Way stars follow a roughly circular path around the galactic center. Just as planets orbit the Sun, stars orbit the galaxy. Since gravity is the force holding stars in their orbit, you can use a star’s speed and distance from the center to determine the mass within its orbit. Not all stars have circular orbits, but they do on average. So you can plot the speed vs distance from the center for known stars, and get what is known as the rotation curve. Measurements of this curve in the Milky Way and other galaxies were the first evidence that galaxies had much more mass than could be accounted for by visible stars, leading to the idea of dark matter.
One of the problems with the rotation curve method is that we can only measure stars to a certain distance. We now know that most of our galaxy’s mass isn’t concentrated at the center, but rather extends outward into a galactic halo. We can estimate the mass of the halo from the rotation curve, but we can also look at the motion of globular clusters.
Globular clusters are bright dense clusters of stars. Since stars within a globular cluster are gravitationally bound, these clusters move around the galaxy like a single object. They are found in a sphere surrounding the Milky Way, so measuring their motion helps us measure the mass of the galactic halo.
Various methods for massing the Milky Way. Credit: Bobylev and Bajkova
To measure the outer region of the galactic halo, we can look at the motion of satellite galaxies such as the Magellanic Clouds. There are about 60 small galaxies within about 1.4 million light-years of the Milky Way. Not all of them are in orbit around our galaxy, but many of them are. Since they lay outside our galactic halo, their orbital motions are determined by all of our galaxy’s mass. The only downside of this approach is that with only a few dozen orbiting galaxies, the result isn’t particularly accurate.
All of these approaches calculate the mass of the Milky Way from orbital motion. There are a few methods that don’t rely on the orbital motion. One of these is to look at the tidal plumes of dwarf galaxies. In the history of our galaxy, there are some globular clusters and dwarf galaxies that strayed too close to the central region of the Milky Way and were ripped apart by tidal forces. The remnants of these galaxies form a stream of stars, such as the Sagittarius stream. By calculating the motion of these streams we can estimate galactic mass.
Another approach is to look at stars leaving our galaxy. Occasionally a star will have a near miss with another star and gain enough velocity to escape our galaxy. Since the escape velocity depends on galactic mass, a statistical measure of escaping stars gives a mass for the galaxy.
Finally, we can look at the local group of galaxies. This includes the Andromeda galaxy and its satellite galaxies. Our local group is gravitationally isolated from more distant galaxy clusters, so looking at the equilibrium state of the local group gives us a handle on its overall mass and the mass of the Milky Way.
Each of these approaches has its own advantages and levels of accuracy. None of them are the final say on their own. In this latest work, the team took a statistical average of various methods and derived what we might call the best value for the mass of our galaxy. The value they determined was a trillion solar masses, give or take a few hundred billion solar masses.
Reference: Bobylev, Vadim V. and Bajkova, Anisa T. “Review of Current Estimates of the Galaxy Mass.” arXiv preprint arXiv:2305.18408 (2023).
The post Exactly How Massive is the Milky Way? appeared first on Universe Today.
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If You Could See Gravitational Waves, the Universe Would Look Like This
Imagine if you could see gravitational waves.
Of course, humans are too small to sense all but the strongest gravitational waves, so imagine you were a great creature of deep space, with tendrils that could extend a million kilometers. As gravitational waves rippled across your vast body, you would sense them squeezing and tugging ever so slightly upon you. And your brilliant mind could use these sensations to create an image in your mind. The ripples of distant supernovae, merging black holes, the undercurrent of the gravitational background. Creation, and destruction, all seen in your mind’s eye.
Perhaps there is such a creature in the vastness of space, but we humans must rely upon our intelligence and engineering. And we may achieve such a vision of the cosmos through a gravitational wave observatory such as the Laser Interferometer Space Antenna, or LISA.
Similar to LIGO, LISA will detect gravitational waves by bouncing laser light along extended arms, measuring the minuscule variations in arms length. But while LIGO has arms just 4 kilometers long, LISA could have arms millions of kilometers long. Where LIGO can detect powerful transient bursts of gravitational waves with frequencies under a kilohertz, such as the mergers of black holes, LISA will detect millihertz waves and will be able to detect not just black hole mergers, but the gradual inspiraling of supermassive black holes and possibly even the remnant gravitational waves of the big bang.
Artist’s impression of the Laser Interferometer Space Antenna (LISA). Credit: ESA
With all this data, astronomers will be able to create a picture of the gravitational wave sky, just as radio astronomers can create images from radio light. If you wonder what the gravitational sky might look like, we now have an idea thanks to a recent study.
The team looked at various known gravitational wave sources such as binary white dwarf, neutron stars, and merging black holes, and calculated the frequencies and magnitudes of their gravitational waves. They then filtered these sources through the estimated limits of what LISA and a second proposed telescope the Advanced MilliHertz Gravitational-wave Observatory (AMIGO) should detect. From this, they assigned colors to various frequency ranges to create a false-color image of the sky. You can see this image above.
We’re still a decade or more away from the launch of LISA, so it will be a while before we can see the real image of the gravitational sky. But that image is out there right now. It ripples through all of us and has every day of our lives. If we’re patient and clever, it’s only a matter of time until we finally see those waves upon our cosmic shore.
Reference: Szekerczes, Kaitlyn, et al. “Imaging the Milky Way with Millihertz Gravitational Waves.” The Astronomical Journal 166.1 (2023): 17.
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Solar Sails Could Reach Mars in Just 26 Days
A recent study submitted to Acta Astronautica explores the potential for using aerographite solar sails for traveling to Mars and interstellar space, which could dramatically reduce both the time and fuel required for such missions. This study comes while ongoing research into the use of solar sails is being conducted by a plethora of organizations along with the successful LightSail2 mission by The Planetary Society, and holds the potential to develop faster and more efficient propulsion systems for long-term space missions.
“Solar sail propulsion has the potential for rapid delivery of small payloads (sub-kilogram) throughout the solar system,” Dr. René Heller, who is an astrophysicist at the Max Planck Institute for Solar System Research and a co-author on the study, tells Universe Today. “Compared to conventional chemical propulsion, which can bring hundreds of tons of payload to low-Earth orbit and deliver a large fraction of that to the Moon, Mars, and beyond, this sounds ridiculously small. But the key value of solar sail technology is speed.”
Unlike conventional rockets, which rely on fuel in the form of a combustion of chemicals to exert an external force out the back of the spacecraft, solar sails don’t require fuel. Instead, they use sunlight for their propulsion mechanism, as the giant sails catch solar photons much like wind sails catching the wind when traveling across water. The longer the solar sails are deployed, the more solar photons are captured, which gradually increases the speed of the spacecraft.
For the study, the researchers conducted simulations on how fast a solar sail made of aerographite with a mass up to 1 kilogram (2.2 pounds), including 720 grams of aerographite with a cross-sectional area of 104 square meters, could reach Mars and the interstellar medium, also called the heliopause, using two trajectories from Earth known as direct outward transfer and inward transfer methods, respectively.
The direct outward transfer method for both the trip to Mars and the heliopause involved the solar sail both deploying and departing directly from a polar orbit around the Earth. The researchers determined that Mars being in opposition (directly opposite Earth from the Sun) at the time of solar sail deployment and departure from Earth would yield the best results for both velocity and travel time. This same polar orbit deployment and departure was also used for the heliopause trajectory, as well. For the inward transfer method, the solar sail would be delivered to approximately 0.6 astronomical units (AU) from the Sun via traditional chemical rockets, where the solar sail would deploy and begin its journey to either Mars or the heliopause. But how does an aerographite solar sail make this journey more feasible?
Image taken by The Planetary Society’s LightSail 2 on 25 November 2019 during its mission orbiting the Earth. The curved appearance of the sails is from the spacecraft’s 185-degree fisheye camera lens, and the image was processed with color-correction along with removal of parts of the distortion. (Credit: The Planetary Society)
“With its low density of 0.18 kilograms per cubic meter, aerographite undercuts all conventional solar sail materials,” Julius Karlapp, who is a Research Assistant at the Dresden University of Technology and lead author of the study, tells Universe Today. “Compared to Mylar (a metallized polyester foil), for example, the density is four orders of magnitude smaller. Assuming that the thrust developed by a solar sail is directly dependent on the mass of the sail, the resulting thrust force is much higher. In addition to the acceleration advantage, the mechanical properties of aerographite are amazing.”
Through these simulations, the researchers found the direct outward transfer method and inward transfer method resulted in the solar sail reaching Mars in 26 days and 126 days, respectively, with the first 103 days being the travel time from Earth to the deployment point at 0.6 AU. For the journey to the heliopause, both methods resulted in 5.3 years and 4.2 years, respectively, with the first 103 days of the inward transfer method also being devoted to the travel time from the Earth to the deployment point at 0.6 AU, as well. The reason the heliopause is reached in a
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Dark Photons Could Be the Key to Both Dark Matter and the Muon Anomaly.
If dark matter exists, then where are the particles?
This single question threatens to topple the standard cosmological model, known as the LCDM model. The CDM stands for cold dark matter, and according to the model makes up nearly 85% of matter in the universe. It should be everywhere, and all around us, and yet every single search for dark matter particles has come up empty. If dark matter particles are real, we know what they are not. We don’t know what they are.
There are lots of ideas, from WIMPs to axions to sterile neutrinos, and none of them have shown up in our detectors. But one of the problems could be that while dark matter particles are everywhere, their particle mass is much higher than we can detect in our particle accelerators and neutrino observatories. If that’s the case, we may never observe them directly. But we might be able to detect the force that allows them to interact.
In particle physics, each fundamental force has one or more carrier bosons. Electromagnetism has the photon, the strong force has the gluons, the weak force has W & Z bosons, the gravitational force the graviton. Dark matter interacts gravitationally, but it also may interact via a dark force, which should have a carrier boson known as the dark photon.
A hypothetical dark photon interaction. Credit: APS/Alan Stonebraker
Dark photons turn up in a generalization of the standard model of particle physics. According to theory, they would interact with dark matter similar to the way photons interact with charged particles. But just as the weak force and electromagnetism are connected as the electroweak force, this dark force and electromagnetism would be connected as a kind of electrodark force. What this means is that regular photons and dark photons could mix slightly, allowing dark matter to interact with regular matter very slightly. Although photons have no mass, dark photons would have mass. This means they would only interact over very short distances, and could quickly decay into other particles. Like the gluons of the strong force, we can’t observe them directly, but we can observe how they cause particles to interact. This is where a new study on dark photons comes in.
The authors analyze the dark photon model in two ways. The first is to use experimental data to constrain the physical parameters of dark photons, such as their mass and how strongly they mix with regular photons. The second is to compare a particle physics model with and without dark photons to key experimental results. In general, the study finds that the dark photon model is a better fit than the standard model, but it’s a particularly good fit for an experiment known as the anomalous magnetic moment of the gluon, or g-2.
The muon is a heavier sibling of the electron, and like the electron, it has an electric charge and a magnetic moment, or g-factor. The value of the muon g-factor is almost, but not exactly, equal to 2. The “not exactly” part, g – 2, is one of the most precisely measured values in particle physics. It is also one of the most precisely calculated values in particle theory. And they don’t agree.
Experiment vs theory for g – 2. Credit: Ryan Postel, Fermilab/Muon g-2 collaboration
Experimentally, g-2 = 0.00233184121. Theoretical calculations put g-2 = 0.00233183620. This is known as the g-2 anomaly and is beyond irksome. If you include dark photon interactions, the theoretical result becomes g-2 = 0.00233183939, which is significantly better. Overall, the dark photon model is preferred over the standard model at 6.5 sigma, which is a very strong result.
All of this is very interesting, but we should add a few caveats. The first is
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