The gravitational waltz of the Milky Way’s satellites

[ This blog is dedicated to tracking my most recent publications. Subscribe to the feed to keep up with all the science stories I write! ]

I’ve started contributing to the Forbes Science page again! This is my first new contribution, relating to the second data release from the Gaia survey telescope. (And if I can be shameless: Forbes pays according to traffic, so the more of you who share and visit and read my stuff, the better they pay me. Ahem.)

Plotting The Three-Dimensional Dance Of Galaxies

A map of the Milky Way’s satellite galaxies, globular clusters, and other objects in orbit. [Credit: ESA/Gaia]

For Forbes:

The European Space Agency’s Gaia telescope is designed to map the position and speed of a billion stars in the Milky Way and its neighboring galaxies. In fact, some of those galaxies are satellites, which whirl around our home galaxy in a complicated dance. Part of Gaia’s mission is to help us understand that dance.

Many of these satellite galaxies actually orbit inside the halo of the mysterious, invisible dark matter that makes up most of the Milky Way’s mass. For that reason, the dance of the satellites tells us about the structure of the Milky Way, along with the shared history and evolution of all the galaxies involved. The Gaia space telescope’s second data release from last week allowed astronomers to map out the positions and motion of stars inside eleven satellite galaxies, along with other star clusters. The result: new estimate on the mass of the Milky Way, and a fully three-dimensional map of nearly 90 objects in orbit around our galaxy.

[Read the rest at Forbes]


Seeing the invisible monster at the Milky Way center

[ This blog is dedicated to tracking my most recent publications. Subscribe to the feed to keep up with all the science stories I write! ]

This is my second print magazine feature for Smithsonian Air & Space Magazine. The first was about gravitational waves, published not long before the LIGO detector found the first gravitational wave signals. The new piece is about the black hole at the center of our galaxy, published just a few months before…well, read the article to see why this is a good time to be writing about that particular black hole.

The First Sighting of a Black Hole

We know one lurks at the center of the Milky Way, but to these astronomers, seeing will be believing

For Smithsonian Air & Space Magazine:

he center of the galaxy doesn’t look like much, even if you’re lucky enough to live in a place where the night sky is sufficiently dark to see the bands of the Milky Way. In visible light, the stars between here and there blur together into a single brilliant source, like a bright beam hiding the lighthouse behind it.

But in other types of radiation—radio waves, infrared, X-rays—astronomers have detected the presence of an object with the mass of four million suns packed into a region smaller than our solar system: a supermassive black hole.

Astronomers call it Sagittarius A*, or Sgr A* (pronounced “sadge A star”) for short, because it’s located (from our point of view) in the Sagittarius constellation. Discovering the Milky Way’s black hole has helped cement the idea that the center of nearly every large galaxy harbors a supermassive black hole. But despite mounting evidence for black holes, we still haven’t seen one directly. [Read the rest at Smithsonian Air & Space Magazine]

The dinosaur-killing dark matter of DOOM!

[ This blog is dedicated to tracking my most recent publications. Subscribe to the feed to keep up with all the science stories I write! ]

A few weeks ago, several news outlets ran stories based on a press release, in which a researcher claimed that dense clumps of dark matter could be responsible for the extinction of dinosaurs. I found this claim dubious, based on what we know about dark matter. Here’s my response.

Did Dark Matter Doom the Dinosaurs?

From Slate:

he history of life on Earth is marked by occasional mass extinctions, events wiping out huge numbers of species. The most famous of these killed off all the dinosaurs (or at least those that hadn’t evolved into birds) 65 million years ago. But the mass extinction that ended the Permian period 250 million years ago was even more dramatic, killing off 90 percent of all species in an astonishingly short amount of time. As yet, the cause of this devastation is unexplained.

Mass extinctions have happened at least five times. (A sixth great extinction currently in progress, but that’s an anomaly because humans are responsible.) Some researchers have tried to figure out whether they’re periodic, recurring after specific time intervals. If they truly do repeat regularly, maybe there’s a common cause for them. [read more on]

The week in review (August 25-31)

The more money we raise to help us go to GeekGirlCon, the more places I will go wearing my Cthulhu hat.

The more money we raise to help us go to GeekGirlCon, the more places I will go wearing my Cthulhu hat.

Welcome to the weekly round-up of stories I wrote this week, wherever they hide.

  • A tour of physics, Angry Birds style (Double X Science): The odds are good that you’ve played Angry Birds, even if (like me) you don’t own a device that will run the game. My colleague Rhett Allain wrote a book for kids, using Angry Birds as an invitation to learn quite a bit about physics, from particle trajectories to cosmology. I reviewed the book for Double X Science.
  • My book-in-progress, Back Roads, Dark Skies, hit a major snag, and its future is unclear. Based on the responses I’ve received, I will not be able to find a publisher without changing the book in an essential way, so I’m feeling a little stuck. So, to show myself (if nobody else) that I’ve accomplished something in the 18 months I’ve been working on the book, I published two excerpts from Chapter 2: Of Bosons and Bison at Galileo’s Pendulum.
  • Microcosmos: My tour of the DZero detector at Fermilab, with a digression on my favorite New Yorker cartoonist.
  • Naming the animals in the particle zoo: The hows and whys of particle detection, in the context of the Tevatron at Fermilab. This excerpt also includes what may be my best joke yet, if I can say that about my own writing.
  • The Milky Way’s black hole, like Cookie Monster, loses more than it eats (Ars Technica): Astronomers have known for many years that our galaxy harbors a supermassive black hole. Yet, it’s a very quiet black hole: the material surrounding it emits very little light compared to other galactic nuclei. A new X-ray observation may hold the key: only about 1 percent of all the material swirling around the black hole is captured, making it a Cookie Monster-level messy eater. (And yes, I’m proud of combining Cookie Monster and black holes in one article.)
  • This doesn’t count as my writing, but I’m joining a number of friends and colleagues at GeekGirlCon in late October for some do-it-yourself science! Well, I’m going if I can afford it; you can help with that by donating to our cause. We’ve already raised more than $400, so I’ve begun photographing myself around the city wearing my Cthulhu hat. If you give us more money, we’ll do even more embarrassing things. You can’t lose.
  • Atmospheric science in a bolt of lightning (Galileo’s Pendulum): Lightning is powerful enough to split molecules into their constituent atoms, and strip electrons away. For a brief moment, lightning can heat air to 30,000° C, more than 5 times the surface temperature of the Sun. An astrophotographer took an amazing snapshot of a lightning flash, with a twist: he used a diffraction grating to split the light into its component colors. The result is that we can identify some of the chemical components of air produced when the molecules and atoms were blasted by the powerful electric discharge.

This week also marked both my parents’ birthdays. Happy birthday, Mom (Monday) and Dad (Friday)!


Our local group of galaxies—known imaginatively as the Local Group—has two huge galaxies: the Milky Way and M31, also known as the Andromeda Galaxy. Both of these galaxies are large enough to have a number of satellites, including the substantial Magellanic Clouds and M33 (Triangulum Galaxy). However, most satellites are dwarf galaxies, very faint and relatively low mass. As a result, a moderately complete census of satellites has proven difficult even for the Milky Way, but what recent observations have found is surprising. In both cases, a number of the satellite galaxies orbit in a single plane, and at least in the case of Andromeda, they orbit in the same direction.

The Pan-Andromeda Archaeological Survey (featuring the diverting acronym PAndAS) was established to provide a high-resolution, large-scale panorama of M31 and its environs. 27 dwarf galaxies that can be unambiguously associated with Andromeda lie within the PAndAS survey region. The astronomers measured the distances and velocities of each of these galaxies, yielding a three-dimensional and dynamical view of the M31 system.

They found 15 of those satellites were arranged along a relatively thin arc from the perspective of Earth, meaning they lie close to a single plane. Further analysis revealed 13 of the 15 galaxies were also moving in a coherent pattern: those “north” of Andromeda were moving away from us, while those “south” were traveling toward us. That indicates a clear rotational pattern; the authors estimated only a 1.4 percent probability of motion like this being random chance. [Read more…]

Why do half of Andromeda’s satellite galaxies orbit in a plane?

Since 1995, a team of astronomers led by Andrea Ghez has been studying the motion of stars near the center of the Milky Way. They just announced the discovery that one of those stars is the closest to the black hole yet, with an orbital period of about 11.5 years—short enough that they’ve been able to track its entire orbit. This could be good news for testing general relativity in a new regime of strong gravity.

One of the earliest identified S-stars was S0-2, with an orbital period of about 16 years. Until the discovery of S0-102, it was the only star with a sufficiently short orbital period to enable a complete three-dimensional reconstruction of its trajectory, which provided the best data on the shape of the black hole’s gravitational influence. [Read more….]

Animation of star motion near the central black hole, based on real infrared observations. [Credit: Andrea Ghez et al./UCLA/Keck]

The darkness at the center of the galaxy

The dark matter problem is famous: about 80% of all mass in the Universe is invisible to light. However, a lot of regular matter is also “missing”, in the sense that we know it exists, but haven’t determined where it’s located. Measurements of the cosmic microwave background have shown how many atoms there are in the Universe, but they aren’t in galaxies. A new observation using the Chandra X-ray Observatory may show that at least some of the missing atoms are in giant hot gas clouds surrounding galaxies.

If the atoms were uniformly distributed around the galaxy, then they would make a sphere over 600 thousand light-years across, about six times the diameter of the Milky Way’s disk. Since the atoms are almost certainly not in a uniform cloud, the actual size of the cloud can’t be known, so the researchers weren’t able to determine whether the oxygen was in the CGM or the IGM. Either way, it is consistent with theoretical predictions. [Read more….]

Where are the missing atoms? All around us!