Universe Today

Pity the poor brown dwarf; a star wannabe. With as little as 3% the mass of the Sun, brown dwarfs don’t have the gravitational pressure needed to ignite fusion in their cores. Instead of blazing bright, they’re dim little objects that smolder quietly for eons. They’re small and dim, and this makes them almost impossible to locate, let alone weigh. But that’s what a team of astronomers have done. They’ve gathered accurate measurements of the lowest mass, free-floating objects ever seen.

When it comes to being a star, mass is everything. Once a star gets down below a certain point – about 7% the mass of the Sun – it doesn’t have enough pass to ignite nuclear fusion in its core. While our Sun’s temperature is nearly 6000 Kelvin, brown dwarfs are only a little hotter than an oven at 700 Kelvin. A typical brown dwarf will put out 1/300,000th the energy of the Sun. Like I said, cool and hard to find.

The most accurate way of determining mass is by looking for a binary object, where a brown dwarf is orbiting another object, like a higher-mass star (or another brown dwarf). Astronomers from Hawaii and Australia did just that.

Think back to your high school physics class. Johannes Kepler first proved in the 17th century that the total mass of any binary system can be determined by precisely measuring the orbit’s size and how long it takes for the two objects to complete one orbital cycle. If you can accurately measure the orbital periods, the mass is easy to calculate.

It’s measuring the orbital periods that was the trick. Here’s Trent Dupuy from the University of Hawaii’s Institute for Astronomy, “these are very challenging measurements, because brown dwarf binaries have tiny separations on the sky and orbit each other very slowly. We needed to obtain the sharpest measurements that are possible with current telescopes to precisely monitor their motion.”

They gathered their data using the 10-meter Keck II Telescope atop Hawaii’s Mauna Kea. The Keck II is built with an adaptive optics system that’s absolutely perfect this kind of task.

The team measured the mass of two brown dwarf binaries. One was composed of two “methane” brown dwarfs – the coolest kind of brown dwarf. The total mass of the two objects was about 6% of the Sun, so 3% of the Sun each. The other pair was a set of warmer, “dusty” brown dwarfs, with 11% the mass of the Sun – so 5.5% each.

Original Source: IFA News Release

My father was a rancher, who had the uncanny ability to accurately estimate the weight of each animal in his herd of cattle simply by looking at them. Today, at the American Astronomical Society meeting in St. Louis, astronomers announced a new, simple way of determining masses of super-massive black holes by just looking at images of galaxies. Dr. Marc Seigar from the University of Arkansas at Little Rock has been studying images available at the Hubble Space Telescope archive site, and looking at the how tightly the galaxy’s arms wrap around itself in relation to the size of the galaxy’s super-massive black hole. “This provides a much simpler method of determining black hole mass,” said Seigar. “You just need an image of a galaxy and you can measure the tightness of the spiral structure. This can easily be applied to distant galaxies, up to 8 billion light-years away.”

Usually astronomers determine masses of super-massive black holes by looking at how fast the stars are moving in the central regions of the galaxies. But that method only works for nearby galaxies. Astronomers have been looking for a new method for galaxies that are father away. This new inexpensive method can use already-existing images, as Seigar has used from the Hubble Site.

Seigar and his team looked at photographs of 27 spiral galaxies including the Milky Way and the Andromeda Galaxy. They observed galaxies with the smallest black holes had spiral arms with angles up to 43 degrees between the arms and the central bulge. Those with the biggest black holes had spiral arms at angles of only 7 degrees between the arms and the bulge.

Seigar said its also possible that the main factor in determining the mass of a super massive black hole is the amount of central concentration of dark matter in a galaxy. “We think dark matter is driving most of the relationships between black holes and their galaxies,” he said. “The masses of these black holes can be determined indirectly from the characteristics of the light emitted from in falling material.”

Seigar will continue to use this method to verify his findings, as well as looking at other aspects. “We have to determine if the relationship between spiral arms and black hole mass evolves over time.”

Source: AAS press conference

It’s not hard to grab someone’s attention when you mention the words “super massive black holes,” “gamma ray bursts,” “cosmic rays,” and ” dark matter.” NASA’s next space telescope will attempt to grab data about some of these high-energy objects in our universe to help us understand their mysteries. GLAST, the Gamma-ray Large Area Space Telescope will use its instruments to study those objects that generate gamma-ray radiation, the most energetic form of radiation we know of, billions of times more energetic than the type of light visible to our eyes. Liftoff for GLAST is set for Thursday, June 5, and the launch window extends from 11:45 a.m. to 1:40 p.m. EDT.

GLAST will reside in a circular, low Earth orbit of about 560 km ( 350 miles ). This orbit was chosen to minimize the effects of charged particles that surround Earth, and which would create additional unwanted background signals in the detectors. At that altitude, the observatory will circle Earth every 90 minutes. In sky-survey mode, GLAST will be able to view the entire sky in just two orbits, or about 3 hours.

The instruments on the GLAST mission are the Large Area Telescope (LAT) and the GLAST Burst Monitor. Lat has a tracker for gamma-ray detection and direction measurement, and can also measure the energies of the rays. The GBM will have two types of scintillators mounted on the sides of the spacecraft to detect electromagnetic waves.

Phil over at Bad Astronomy has a couple of very nice (and fun) videos with info about GLAST (one starring Phil himself), but these are the major goals of GLAST:

• Explore the most extreme environments in the Universe, where nature harnesses energies far beyond anything possible on Earth.
• Search for signs of new laws of physics and what composes the mysterious Dark Matter.
• Explain how black holes accelerate immense jets of material to nearly light speed.
• Help crack the mysteries of the stupendously powerful explosions known as gamma-ray bursts.
• Answer long-standing questions across a broad range of topics, including solar flares, pulsars and the origin of cosmic rays.

GLAST should provide some very interesting data about these spectacular and remarkable objects in our universe, and will create a full-sky map of gamma radiation.

Kapla GLAST!

Source: GLAST site