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Problems posed by massive stars

Massive stars affect their surroundings profoundly — and far more than any of their stellar brethren. Astronomers still ponder many mysteries that surround these rare beasts.
The Arches Cluster is the densest star cluster in our galaxy. Astronomers used the Hubble Space Telescope to weigh the stars there and found the heftiest to be around 150 solar masses — a likely upper limit to the biggest stars that can form in the current universe.
NASA, ESA, and Donald Figer (STScI)
If you were an astronomer seeking to make your name — or simply to advance our knowledge of the cosmos — what would you study? These questions were at least part of the motivation when Cássio Barbosa of Brazil's University of São Paulo and Donald Figer of the Space Telescope Science Institute in Baltimore surveyed observers and theorists to see what they believed were the most pressing problems concerning massive stars. The list serves as a valuable reference, particularly for scientists entering the field.

In "Live fast, die young" (April 2006), Astronomy examined 10 top questions about massive stars:

1) How far away are massive stars?
2) How do massive stars form?
3 What is a star's maximum mass?
4) What role did massive stars play after the Big Bang?
5) How do massive binary stars form?
6) Can planets form around massive stars?
7) What role do rotation and magnetic fields play in massive stars?
8) How do single massive stars evolve?
9) How do massive binaries form?
10) How do massive stars die?

The field of massive stars is bigger than just 10 questions, however. Here, we present 10 more questions that keep astronomers awake at night.
1) How did the universe's first massive stars form when no heavy elements existed?
Theory tells us the first stars in the universe were massive — each probably contained several hundred times the Sun's mass. These stars managed to form despite a dearth of heavy elements. Within minutes of the Big Bang, the only elements around were hydrogen, helium, and a trace of lithium. Yet, as a star starts to form by contracting, heat builds up, and hot gas tends to expand, not contract further. In today's universe, atoms and molecules effectively cool the gas and allow the collapse to continue. Hydrogen and helium, however, don't perform this function well. Molecular hydrogen likely played a significant role in the young universe, but exactly how early, massive stars formed remains a mystery.
2) How much metallicity — the percentage of elements heavier than helium — is needed before star formation can give rise to a full range of stellar masses?
Theorists are convinced that the universe's first stars must have had masses hundreds of times that of the Sun. Only with masses this big can a contracting gas cloud compensate for the limited amount of cooling provided by hydrogen molecules. The current universe creates stars with masses all the way down to about 8 percent that of the Sun. The metals produced in the nuclear furnaces of the big, early stars — and ultimately expelled in supernova explosions — played the key role in allowing smaller stars. The question astronomers now ask is how much metal enrichment was needed before stars with a range of masses could form.
3) What is the stellar census of giant molecular clouds?
In the current universe, all massive-star formation appears to take place in giant molecular clouds. Yet, these clouds are so thick with gas and dust that astronomers have a hard time seeing into them. Do these molecular clouds form stars with the same mass distribution seen in older, more evolved clusters? Deep imaging and spectroscopic observations in the infrared part of the spectrum should be able to answer this question in the next several years.
4) How fast do stars with different levels of heavy elements lose mass?
Presumably, the metallicity of a star plays a role in the mass-loss rate. This should be particularly true for massive stars, including supergiants and Wolf-Rayet stars, which always lose mass at a faster rate than smaller stars. And a related question: Do the stellar winds that carry away a star's mass blow evenly around the whole star, or is it concentrated at the poles or equator?
5) In a binary system where matter moves from one component to the other, what fraction of the mass does the receiving star retain?
The more mass a star contains, the faster it evolves. Eventually, a star will become a bloated red giant or supergiant. If such a star belongs to a binary system, and its component lies close enough, the evolved star's outer layers will be pulled more strongly by the companion star's gravity, and mass will flow from the bigger to the smaller star. Such mass transfer happens in many systems, including X-ray binaries and type Ia supernovae. The bigger star can lose lots of material this way, but no one knows what fraction the smaller star can hold onto.
6) What is the stellar content and rate of star formation in the Milky Way Galaxy's central bar and molecular ring?
Radio surveys of our galaxy show it has a central bar some 27,000 light-years long and a prominent molecular ring, centered on the core, with a radius of about 15,000 light-years. Yet, radio observations can say little about the kinds of stars that make up these structures, the rate at which stars are forming there, or the role played by massive stars. This problem ranks among the most serious in galactic astronomy.
7) What is the site of the r-process?
The nuclear furnaces in massive stars create all the elements up through iron. Heavier elements all formed by a process known as neutron capture — either slowly (the s-process) or rapidly (the r-process). Each process produces roughly half of the total elements, and the r-process alone creates all elements heavier than bismuth (with an atomic mass of 209). Astronomers are convinced the only place where rapid neutron capture can occur is in massive stars, and likely most of it happens during supernova explosions. Yet exactly where in the supernova can't be pinned down, in large part because astronomers don't yet fully understand the explosions themselves.
8) What is the minimum mass above which a massive star will end its life as a black hole instead of a neutron star?
Stars born with less than about 8 solar masses lose enough mass during their lives to end up as white dwarfs — Earth-size objects containing the approximate mass of the Sun. But if a star starts out with more than 8 solar masses, it likely will end its life as a supernova, violently ejecting its outer layers. The stellar core left behind will be either a neutron star — with a couple of solar masses packed into a sphere the size of a city — or a black hole — an object so dense even light can't escape. No one has yet figured out where the cutoff lies between a star that eventually will become a neutron star and one that will collapse to a black hole. The star's metal content may play a role, as well as whether the star belongs to a binary system.
9) Do massive stars explode when they form a black hole?
Conventional wisdom says a massive star explodes once it exhausts its nuclear fuel and iron builds up in its core. Iron doesn't produce energy when it fuses, so the star's core collapses and — somehow — the collapse generates a shock wave that blows the rest of the star apart. In the past few years, it has become obvious that some stars explode with even greater force, creating hypernovae and gamma-ray bursts. But some theorists think the most massive stars may end their lives by collapsing directly to a black hole, without any explosion. The only way astronomers could detect such an event would be through neutrinos or gravitational waves emitted during the process, something improved technology may allow in the future.
10) How does a supernova interact with the interstellar medium?
The outer layers of an exploding star get ejected violently into surrounding space. This ejecta enriches the interstellar medium with heavy elements, but no one knows how the ejecta eventually merges with the material in molecular clouds. The supernova's shock wave also can trigger interstellar clouds to collapse and form new stars. The question astronomers ask most often on this front: How big a role do supernovae play in a galaxy's overall star formation?


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