Tag Archives: galaxies

Mapping the Unknown, Times Three

I took a short break from other projects this weekend. Poking around, I found three particularly lovely science stories, which I thought some readers might enjoy as well. They all involve mapping, but the distances involved are amazingly different.

Starbirth Near to Home

The first one has been widely covered in the media (although a lot of the articles were confused as to what is new news and what is old news.) Our galaxy is full of stars, but also of gas (mainly hydrogen and helium) and dust (tiny grains of material made mainly from heavier elements forged in stars, such as silicon). That gas and dust, referred to as the “interstellar medium”, is by no means uniform; it is particularly thin in certain regions which are roughly in the shape of bubbles. These bubbles were presumably “blown” by the force of large stellar explosions, i.e. supernovas, whose blast waves cleared out the gas and dust nearby.

It’s been known for several decades that the Sun sits near the middle of such a bubble. The bubble and the Sun are moving relative to one another, so the Sun’s probably only been inside the bubble for a few million years; since we’re just passing through, it’s an accident that right now we’re near its center. Called simply the “Local Bubble”, it’s an irregularly shaped region where the density of gas and dust is 1% of its average across the galaxy. If you orient the galaxy in your mind so that its disk, where most of the stars lie, is horizontal, then the bubble stretches several hundred light-years across in the horizontal direction, and is elongated vertically. [For scale: a light-second is 186,000 miles or 300,000 km; the Sun is 8 light-minutes from Earth; the next-nearest star is 4 light-years away; and our Milky Way galaxy is about 100,000 light years across.] It’s been thought for some time that this bubble was created some ten to twenty million years ago by the explosion of one or more stars, probably siblings that were born close by in time and in space, and which at their deaths were hundreds of light-years from the Sun, far enough away to do no harm to Earthly life. [For scale: recall the Sun and Earth are about 5 billion years old.]

Meanwhile, it’s long been suggested that explosion debris from such supernovas can sweep up gas and dust like a snowplow, and that the compression of the gas can lead it to start forming stars. It’s a beautiful story; large stars live fast and hot, and die young, but perhaps as they expire they create the conditions for the next generation. [A star with 40 times the raw material as the Sun has will burn so hot that it will last only a million years, and most such stars die by explosion, unlike Sun-like stars.]

If this is true, then in the region near the Sun, most if not all the gas clouds where stars are currently forming should lie on the current edge of this bubble, and moreover, all relatively young stars, less than 10 million years old, should have formed on the past edge of the bubble. Unfortunately, this has been hard to prove, because measuring the locations, motions and ages of all the stars and clouds isn’t easy. But the extraordinary Gaia satellite has made this possible. Using Gaia’s data as well as other observations, a team of researchers (Catherine Zucker, Alyssa A. Goodman, João Alves, Shmuel Bialy, Michael Foley, Joshua S. Speagle, Josefa Groβschedl, Douglas P. Finkbeiner, Andreas Burkert, Diana Khimey & Cameren Swiggum) has claimed here that indeed the star-forming regions lying within a few hundred light-years of the Sun all lie on the Local Bubble’s surface, and that nearby stars younger than ten million years were born on the then-smaller shell of the expanding bubble. Moreover they claim that the bubble probably formed 14-15 million years ago, at a time when the Sun was about 500 light-years distant.

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Looking for Signs of Dark Matter at the Milky Way’s Center

There is going to be some amount of debate regarding dark matter in the next few weeks, so I’ve written an article on one of the best ways to go looking for new signs of dark matter out in space.

The reason we are almost entirely convinced that the universe has lots of matter that doesn’t shine is that we can see many signs of its gravitational effects — for instance, its effect on the motions of stars within galaxies, its ability to bend light a la Einstein, etc.  It’s almost certain that most of a galaxy is dark matter.  And over the years we’ve convinced ourselves this dark matter almost certainly can’t be made from any type of particle that we already know about.

But to learn more about what it is, we need to find signs of some of its non-gravitational effects, if it has any.  One possibility is that dark matter particles, if and when they collide, might annihilate into ordinary known particles.  If those known particles are photons, we might be able to detect them.  A good way to look for them would be to point a suitable telescope toward the center of the Milky Way, our galaxy, which is one place where we expect dark matter particles to be especially numerous, and collisions among them to be especially common.

In the article I just finished, I explain how this can be done.  One goes looking for photons from the galactic center, makes a plot of the number of photons observed at a particular energy, and looks for a bump in the plot — an exceptional number of photons with the same energy.

And the reason I’m doing this now is that there is a new paper claiming that a signal of this type may have been seen (with a claimed significance of 3.3 standard deviations, after including the look-elsewhere effect.)  This is a paper by a theorist, analyzing publicly available data taken by the experimental group that operates the Fermi Large Area Telescope satellite.  One should note that the record of theorists making discoveries using experimentalists’ data is very poor.  Typically there are either detector-related or statistics-related issues that theorists screw up.  And there are risks of bias — I am not yet sure whether the rather sophisticated analysis method used by this theorist was chosen in a blinded fashion.  [For instance, did he choose his method first and then look at the data, or did he already know there was a hint of a peak in the data before he started designing his method?] So I would be skeptical of this claim for now.  (And the theorist, knowing he’s out on a limb, was careful [and wise] to put the word “Tentative” in his title.)   However, stranger things have happened, so I wouldn’t dismiss this claim out of hand either, at least not until the Fermi experimentalists tell us that in their opinion the theorist over-estimated the statistical significance of this particular bump.  We’ll be looking forward to what they have to say.

I’ll have a few more details about this for you soon.