Hubble Fireworks: The Antennae Galaxies in Collision

Two galaxies are squaring off in Corvus and the latest pictures can be viewed here. 

When two galaxies collide, the stars that compose them usually do not. 

That's because galaxies are mostly empty space and, however bright, stars only take up only a small amount of that space. 

During the slow, hundred million year collision, one galaxy can still rip the other apart gravitationally, and dust and gas common to both galaxies does collide. 

In this clash of the titans, dark dust pillars mark massive molecular clouds are being compressed during the galactic encounter, causing the rapid birth of millions of stars, some of which are gravitationally bound together in massive star clusters.

Credit: Hubble Legacy Archive, NASA, ESA

Black holes found in globular star clusters, upsetting 40 years of theory

The black hole above was discovered in the M62 star cluster, which is 23,000 light years away from Earth. 

These star clusters contain some of the oldest stars in the galaxy.

A Texas Tech University astrophysicist was part of a team of researchers that discovered the first examples of black holes in globular star clusters in our own galaxy, upsetting 40 years of theories against their possible existence.

Tom Maccarone, an associate professor of physics, said the team detected the existence of the black holes by using an array of radio telescopes to pick up a certain type of radio frequency released by these black holes as they eat a star next to them.

The results were published in the Astrophysical Journal and featured in the National Radio Astronomy Observatory's ENews news bulletin.

Globular star clusters are large groupings of stars thought to contain some of the oldest stars in the universe.

In the same distance from our sun to the nearest neighbour, Proxima Centauri, its nearest neighbor, these globular star clusters could have a million to tens of millions of stars, Maccarone said.

Tom Maccarone

"The stars can collide with one another in that environment," Maccarone said. "The old theory believed that the interaction of stars was thought to kick out any black holes that formed. They would interact with each other and slingshot black holes out of the cluster until they were all gone."

He compared it to water vapor coming off a hot cup of coffee. As some water molecules get hot enough to turn to steam, they are let go from their environment to float off into the atmosphere even though the coffee may be below the boiling temperature of water.

The old theory stated that the stars would kick the black holes out in the same fashion – occasionally, some black holes would have enough energy to escape the cluster, and gradually, they all would leave.

While the theory may still be displaced, Maccarone said it might still be somewhat true. Black holes might still get kicked out of globular star clusters, but at a much slower rate than initially believed.

Radio image (left) and x-ray image (right). The yellow circle shows the black hole found in the M62 star cluster in our Galaxy. The red circle denotes a neutron star close by.

In 2007, Maccarone made the first discovery of a black hole in a globular star cluster in the neighboring NGC4472 galaxy. But rather than finding it by using radio waves, Maccarone found it by seeing an X-ray emission from the gas falling into the black hole and heating up to a few million degrees.

"Six years ago I had made the first discoveries in other galaxies," he said. "It's surprisingly easier to find them in other galaxies than in our own, even though they're a thousand times as far away as these in our own galaxy are."

This year, he and his team discovered two examples of globular star clusters in our own galaxy which host black holes by finding radio emission by using the Very Large Array of radio telescopes in New Mexico.

"As the black hole eats a star, these jets of material are coming out," he said.

"Most of the material falls into the black hole, but some is thrown outwards in a jet. To see that jet of material, we look for a radio emission. We found a few radio emissions coming from this globular star cluster that we couldn't explain any other way."

Maccarone said seeing black holes in globular clusters may provide a way for them to get close enough to one another to merge into bigger black holes.

"These mergers may produce the 'ripples in spacetime' we call gravitational waves," he said. "Trying to detect gravitational waves is one of the biggest problems in physics right now, because it would be the strongest test of whether Einstein's theory of relativity is correct."

More information: iopscience.iop.org/0004-637X/

NASA Hubble Watches Star Clusters on a Collision Course

This is a Hubble Space Telescope image of a pair of star clusters that are believed to be in the early stages of merging.

The clusters lie in the gigantic 30 Doradus nebula, which is 170,000 light-years from Earth.

The Hubble observations, made with the Wide Field Camera 3, were taken Oct. 20-27, 2009.

The blue colour is light from the hottest, most massive stars; the green from the glow of oxygen; and the red from fluorescing hydrogen.

Image Credit: NASA, ESA, R. O'Connell (University of Virginia), and the Wide Field Camera 3 Science Oversight Committee

Astronomers using data from NASA's Hubble Space Telescope have caught two clusters full of massive stars that may be in the early stages of merging.

The clusters are 170,000 light-years away in the Large Magellanic Cloud, a small satellite galaxy to our Milky Way, a small satellite galaxy to our Milky Way.

What at first was thought to be only one cluster in the core of the massive star-forming region 30 Doradus (also known as the Tarantula Nebula) has been found to be a composite of two clusters that differ in age by about one million years.

The entire 30 Doradus complex has been an active star-forming region for 25 million years, and it is currently unknown how much longer this region can continue creating new stars. Smaller systems that merge into larger ones could help to explain the origin of some of the largest known star clusters.

Lead scientist Elena Sabbi of the Space Telescope Science Institute in Baltimore, Md., and her team began looking at the area while searching for runaway stars, fast-moving stars that have been kicked out of their stellar nurseries where they first formed.

"Stars are supposed to form in clusters, but there are many young stars outside 30 Doradus that could not have formed where they are; they may have been ejected at very high velocity from 30 Doradus itself," Sabbi said.

She then noticed something unusual about the cluster when looking at the distribution of the low-mass stars detected by Hubble.

It is not spherical, as was expected, but has features somewhat similar to the shape of two merging galaxies where their shapes are elongated by the tidal pull of gravity.

Hubble’s circumstantial evidence for the impending merger comes from seeing an elongated structure in one of the clusters, and from measuring a different age between the two clusters.

According to some models, the giant gas clouds out of which star clusters form may fragment into smaller pieces.

Once these small pieces precipitate stars, they might then interact and merge to become a bigger system. This interaction is what Sabbi and her team think they are observing in 30 Doradus.