Cosmic Waves

When massive stars forms, they generate powerful winds which stream through their surroundings. Just like winds on Earth generate waves as they flow over water, these stellar waves apparently form waves in the surrounding gas which were recently observed. You can read more about it here, watch a video showing it here, and in its full scientific glory here.

How much mass do you need....

... for a star to form a black hole? Well, apparently more than previously thought.
When massive stars run out of fuel, their Iron core collapses under its own gravity and turns into a neutron star. If the energy released in this collapse is enough to expel the outer layers of the star, you are left with a neutron star. If too much more material falls onto the neutron star before the released energy blows away the rest of the star, you have a black hole. Stars with an initial mass more that 40 times that of the Sun were expected to be well above the threshold where you make a black hole. However, nature appears to have other ideas, as you can read about here and here and here. Maybe such stars lose enough mass before they explode that the initial collapse is sufficient. Hopefully we'll find out soon.

The birth of the most massive stars

The current picture of star formation looks like this: small sections of a giant molecular cloud collapse under their own gravitational attraction (or something like that), and the collapsing gas and dust forms a star. This stops once either there is no more gas and dust or the light output of the star (luminosity) is sufficiently strong to blow away the surrounding gas and dust. Massive stars are so luminous that they stop this process before enough material can collapse to form them, a problem for models. As a result, people thought that maybe massive stars from from the merger of lower mass stars. Not so, say recent observations of a massive star forming that has a dusty disk around it and is expelling material - just as observed around single low mass stars as they form as you can read here or here or here or here or in its full detail here (subscription required. Sorry.)

So how do they form, that is still a mystery. But form directly they appear to do.

Baby massive stars...

Aren't they so cute? Go here to see a recent image of a stellar nursery in the Milky Way, where the most massive star in our galaxy may reside.

Description of January 7 Radio Show: Milky Way Galaxy

Long available here, below is a description of the January 7th episode of the radio show, where I continue the "Tour of the Universe" with a description of the Milky Way:

• Galactic Center and Galactic Plane - At the very center of the Milky Way, there is believed to a black hole which has a mass approximately a million or so times that of the Sun called Sgr A*. Pretty strong evidence for the existence of such an object comes from studying the orbit of stars very close to Sgr A* (link). Measuring the mass of similar black holes believed to be at the centers of other galaxies is much more difficult since there you can't resolve the orbits of individual stars. Other methods which have been proposed are the tightness of their spiral arms (link), the temperature of the hot, X-ray emitting gas surrounding the galaxy (if it is a galaxy without much ongoing star formation; link). Surrounding Sgr A* is a disk of stars including the Sun, referred to as the Galactic Plane. They stars orbits Sgr A*, and a new, precise measurement of the rotation of stars in the Milky Way was recently made using a particular class of pulsating stars called Cepheids - which have also been used to measure the distance to other galaxies (link). The spiral arms in the Milky Way are not believed to concentration of particular stars, but where a significant number of stars are born "at once" - which is why they appear brighter than other parts of the Milky Way and contain a vast majority of the most massive (and therefore, very short lived) stars in the Galaxy. There is evidence that our Sun has traveled a considerable distance from its birth site in the Milky Way. Since young stars are clustered along spiral arms, and young, massive stars are the dominant source of ultraviolet radiation in a galaxy, ultraviolet images like the one the Swift satellite recently made of M33 (link), are good ways of studying the spiral structure in a galaxy. Spiral arms, and other features of the galactic disks make the results of gravitational interactions between galaxies (link). The Milky Way also contains clusters of stars, the densest and oldest of which are called "globular clusters", which are believed to be stars formed at the same time out of the same cloud of gas. A recent study of the a particular globular cluster measured its age using three different methods and got three different answers - a bit of a puzzle (link). As discussed on a previous show, the most massive stars in the Milky Way are believed to end their life in a supernova explosions, which actually plays a very important role in the properties of the gas which fills the Galactic plane of the Milky Way. Recent Chandra and Very Large Array observations have discovered the remnant of the most recent supernova explosion in the Milky Way, believed to have occurred only 140 years ago (link). The material released in these explosion expand with a very high velocity, heating the surrounding gas to very high temperatures and compressing into a thin shell of material which make for lovely Hubble images - and very interesting science (link). The hot gas and cosmic rays produced in the interaction between the material ejected in a supernova and its surrounding might explain the flow of gas out of the Milky Way's Galactic Plane, and carve bubbles out of nearby cold gas as observed in the Tarantula Nebula in the Large Magellanic Cloud (link). This is believed to explain why clusters of young stars are often surrounding by "holes" in gas, though this is not the case for nearby dwarf galaxy IC 2574. In most galaxies like the Milky Way, ionized hydrogen is only found in the center of these galaxies, most likely the result of star formation occurring only in certain regions and not distributed uniformly around a galaxy (link). Spitzer observations of a nearby spiral galaxy M101 shows that organic molecules are only present towards its center and not its edges (link; image. In the Milky Way, most star formation is currently taking place near the center of the galaxy, but not so in M83 where GALEX recently observed star formation at the outer edge (link)
• Wednesday Morning Astronomer: I understand and don't necessarily disagree with his point in this article, but astronomers did not "cavalierly" come up with the idea of Dark Matter - it was first proposed to explain some observations in the 1930s I believe, did not gain acceptance for at least 30 years, and still bothers many people, dark matter has been "located" (though not yet explained), the every controversial "dark flow" observed in distance galaxy clusters might be due to an unknown force, but not one outside our universe but one important on distances greater than the speed of light times the age of the universe which defines the "observable universe." I know that is pretty subtle, but it is an important distinction.
• Calendar of upcoming Astronomy/Science events in the greater Poughkeepsie/New York City area.
• Globular Clusters and Galactic Halo: Outside the Galactic Plane there are dense concentrations of millions of old stars referred to as globular clusters. Recent observations suggest that, even though they are "only" 9-13 billion years old, the structure of stars inside globular clusters is still evolving (link). It is though the some globular clusters are actually the remnants of the centers of dwarf galaxies which have been absorbed by the Milky Way. Since such galaxies are also believed to have black holes at their center, the presence of a massive black hole (much more massive than the Sun) might be proof this occurring. Such a black hole might have been found in globular cluster Omega Centauri (link), which - unlike other globular clusters - also contains dust (link). A similar process might explain the large number of globular clusters in M87, the giant elliptical galaxy in the center of the Virgo cluster - it "stole" them from lower mass galaxies which got too close (link). By measuring the velocity of the stars in the halo of the Milky Way, the diffuse "cloud" of stars which surrounds the Galactic Plane, it is possible to estimate the total mass of our galaxy. A recent such measurement suggests a lower mass than previously estimated (link). How the Milky Way's halo got there is an open, and interesting question. The two possibilities are that the stars formed out of the same cloud of gas that collapsed to form the Milky Way, or that it is the remnants of galaxies which have merged into the Milky Way. Recent studies of the structure of the halo suggest the second possibility, as do the presence of streams of stars in the halos of two nearby galaxies (here). It is expected that there are stars in the vast empty space between galaxies, and astronomers are searching for them (link). Last, but not least, it appears that most of the mass of galaxies is not in stars, gas, or dust, but in "dark matter" - this mysterious stuff that has mass but doesn't seem to produce light. Dark Matter is present in the dwarf galaxies which orbit the Milky Way - in fact, one of these has the highest ratio of dark to "normal" matter of any known object (link), and the galactic plane might be enclosed in a larger disk of dark matter(link). The distribution of dark matter in a galaxy is not expected to be entirely smooth but contain clumps and streams (link) which might be measurable using the motion of nearby stars in the night sky. An alternative to dark matter is the Newton's equation for gravity is wrong on very large distances (called MOdified Newtonian Dynamics, or MOND), which can do a good job reproducing the orbit of dwarf galaxies around the Milky Way (link). There are many fewer known dwarf galaxies around the Milky Way than expected, a problem for our current understanding of galaxy formation, though possible solutions have been suggested (link)

Thank you very much listening, and please email or post below any questions, comments, or concerns you might have.

October 15th Radio Show: Star Formation, Planet Formation, and Massive Stars

Continuing my recap of topics I discussed this summer, here is the October 15th episode of the radio show, where I discuss the latest results concerning star and planet formation, massive stars, in addition to the usual Astronomy news and calendar. I'll post a more detailed description later, but in the mean time, enjoy! Comments and concerns are always appreciated, and please leave any you have below.