Promising new method to measure star age

Artist's conception of a hypothetical exoplanet. Gyrochronology is a promising new method to learn the ages of isolated stars, including all stars known to have planets. David A. Aguilar (CfA)

By Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts
Published: May 23, 2011

How can we tell if a star is 1 billion or 10 billion years old? Astronomers may have found a solution — measuring the star's spin. "A star's rotation slows down steadily with time, like a top spinning on a table, and can be used as a clock to determine its age," said astronomer Soren Meibom from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Knowing a star's age is important for many astronomical studies and, in particular, for planet hunters. With the bountiful harvest from NASA's Kepler spacecraft (launched in 2009) adding to previous discoveries, astronomers have found nearly 2,000 planets orbiting distant stars. Now, they want to use this new zoo of planets to understand how planetary systems form and evolve and why they are so different from each other.

"Ultimately, we need to know the ages of the stars and their planets to assess whether alien life might have evolved on these distant worlds," said Meibom. "The older the planet, the more time life has had to get started. Since stars and planets form together at the same time, if we know a star's age, we know the age of its planets, too."

Learning a star's age is relatively easy when it's in a cluster of hundreds of stars that all formed at the same time. Astronomers have known for decades that if they plot the colors and brightnesses of the stars in a cluster, the pattern they see can be used to tell the cluster's age. But this technique only works on clusters. For stars not in clusters (including all stars known to have planets), determining the age is more difficult.

Using the unique capabilities of the Kepler space telescope, Meibom and his collaborators measured the rotation rates for stars in a 1-billion-year-old cluster called NGC 6811. This new work nearly doubles the age covered by previous studies of younger clusters. It also significantly adds to our knowledge of how a star's spin rate and age are related.

If a relationship between stellar rotation and age can be established by studying stars in clusters, then measuring the rotation period of any star can be used to derive its age — a technique called gyrochronology. For gyrochronology to work, astronomers first must calibrate their new "clock."

They begin with stars in clusters with known ages. By measuring the spins of cluster stars, they can learn what spin rate to expect for that age. Measuring the rotation of stars in clusters with different ages tells them exactly how spin and age are related. Then by extension, they can measure the spin of a single isolated star and calculate its age.

To measure a star's spin, astronomers look for changes in its brightness caused by dark spots on its surface — the stellar equivalent of sunspots. Any time a spot crosses the star's face, it dims slightly. Once the spot rotates out of view, the star's light brightens again. By watching how long it takes for a spot to rotate into view, travel across the star, and then disappear out of view again, scientists learn how fast the star is spinning.

The changes in a star's brightness due to spots are very small, typically a few percent or less, and become smaller the older the star. Therefore, the rotation periods of stars older than about half a billion years can't be measured from the ground where Earth's atmosphere interferes. Fortunately, this is not a problem for the Kepler spacecraft. Kepler was designed specifically to measure stellar brightnesses very precisely in order to detect planets, which block a star's light ever so slightly if they cross the star's face from our point of view.

To extend the age-rotation relationship to NGC 6811, Meibom and his colleagues faced a herculean task. They spent 4 years painstakingly sorting out stars in the cluster from unrelated stars that just happened to be seen in the same direction. This preparatory work was done using a specially designed instrument, called the Hectochelle, mounted on the MMT telescope on Mt. Hopkins in southern Arizona. Hectochelle can observe 240 stars at the same time, allowing the researcher to observe nearly 7,000 stars over 4 years. Once they knew which stars were the real cluster stars, they used Kepler data to determine how fast those stars were spinning.

They found rotation periods ranging from 1 to 11 days (with hotter, more-massive stars spinning faster), compared to the 30-day spin rate of our Sun. More importantly, they found a strong relationship between stellar mass and rotation rate, with little scatter. This result confirms that gyrochronology is a promising new method to learn the ages of isolated stars.

The team now plans to study other, older star clusters to continue calibrating their stellar "clocks." Those measurements will be more challenging because older stars spin slower and have fewer and smaller spots, meaning that the brightness changes will be even smaller and more drawn out. Nevertheless, they feel up to the challenge.

"This work is a leap in our understanding of how stars like our Sun work. It also may have an important impact on our understanding of planets found outside our solar system," said Meibom.

Turbulence from large black holes halts star formation

Snapshot of gas temperatures in a three-dimensional computer simulation of a cool-core cluster. The blue ring shows the cool gas accreting onto the central black hole disk; the red and yellow jets show the hot gas ejected by this disk. Older bubbles from an earlier outburst are visible on the far left and right sides of the image. Turbulence generated by the jets mixes the hot and cool material together, which stabilizes further accretion and allows the cluster to perform its remarkable balancing act. E. Scannapieco/ M. Brueggen / ASU Fulton High Performance Computing Initiative

New simulations reveal that turbulence created by jets of material ejected from the disks of the universe's largest black holes is responsible for halting star formation. Evan Scannapieco, an assistant professor in the School of Earth and Space Exploration at Arizona State University (ASU) and Marcus Brueggen, professor of Jacobs University in Bremen, Germany, presented the new model.

We live in a hierarchical universe where small structures join larger ones. Earth is a planet in our solar system, the solar system resides in the Milky Way Galaxy, and galaxies combine into groups and clusters. Clusters are the largest structures in the universe. Researchers have long known that the gas in the centers of some galaxy clusters cools and condenses rapidly, but were puzzled why this condensed gas did not form into stars. Until recently, no model existed that successfully explained how this was possible.

Scannapieco has spent much of his career studying the evolution of galaxies and clusters. "There are two types of clusters — cool-core clusters and non-cool core clusters," he said. "Non-cool core clusters haven't been around long enough to cool, whereas cool-core clusters are rapidly cooling, although by our standards they are still very hot."

X-ray telescopes have revolutionized our understanding of the activity occurring within cool-core clusters. Although these clusters can contain hundreds or thousands of galaxies, they are mostly made up of a diffuse, but very hot gas known as the intracluster medium. This intergalactic gas is only visible to X-ray telescopes, which are able to map out its temperature and structure. These observations show that the diffuse gas is rapidly cooling into the centers of cool-core clusters.

At the core of each of these clusters is a black hole, billions of times more massive than the Sun. Some of the cooling medium makes its way down to a dense disk surrounding the black hole, some of it goes into the black hole itself, and some of it is shot outward. X-ray images clearly show jet-like bursts of ejected material, occurring in regular cycles.

But why were these outbursts so regular, and why did the cooling gas never drop to colder temperatures that lead to the formation of stars? Some unknown mechanism was creating an impressive balancing act.

"It looked like the jets coming from black holes were somehow responsible for stopping the cooling," said Scannapieco, "but until now no one was able to determine how exactly."

Scannapieco and Brueggen used the enormous supercomputers at ASU to develop their own three-dimensional simulation of the galaxy cluster surrounding one of the universe's biggest black holes. By adapting an approach developed by Guy Dimonte at Los Alamos National Laboratory and Robert Tipton at Lawrence Livermore National Laboratory, Scannapieco and Brueggen added the component of turbulence to the simulations, which was never accounted for in the past.

And that was the key ingredient.

Turbulence works in partnership with the black hole to maintain the balance. Without turbulence, jets coming from around the black hole would grow stronger and stronger, and the gas would cool catastrophically into a swarm of new stars. When turbulence is accounted for, the black hole not only balances the cooling, but also goes through regular cycles of activity.

"When you have turbulent flow, you have random motions on all scales," said Scannapieco. "Each jet of material ejected from the disk creates turbulence that mixes everything together."

Scannapieco and Brueggen's results reveal that turbulence acts to effectively mix the heated region with its surroundings so that the cool gas can't make it down to the black hole, thus preventing star formation.

Every time some cool gas reaches the black hole, it is shot out in a jet. This generates turbulence that mixes the hot gas with the cold gas. This mixture becomes so hot that it doesn't accrete onto the black hole. The jet stops and there is nothing to drive the turbulence so it fades away. At that point, the hot gas no longer mixes with the cold gas, so the center of the cluster cools, and more gas makes its way down to the black hole.

Before long, another jet forms and the gas once again is mixed together.

"We improved our simulations so that they could capture those tiny turbulent motions," said Scannapieco. "Even though we can't see them, we can estimate what they would do. The time it takes for the turbulence to decay away is exactly the same amount of time observed between the outbursts."

July 14, 2009

Dark Energy Star

A dark-energy star is a hypothetical compact astrophysical object, which a minority of physicists feel might constitute an alternative explanation for observations of astronomical black hole candidates. Dark energy is invisible to the human eye; however, it can be tracked with difficulty by gamma astronomy.

The concept was proposed by physicist George Chapline. The theory states that infalling matter is converted into vacuum energy or dark energy, as the matter falls through the event horizon. The space within the event horizon would end up with a large value for the cosmological constant and have negative pressure to exert against gravity. There would be no information-destroying singularity.

In March 2005, physicist George Chapline claimed that quantum mechanics makes it a "near certainty", that black holes do not exist and are instead dark energy stars. The dark energy star is a different concept than that of a gravastar.

Dark-energy stars were first proposed, because in quantum physics, absolute time is required; however, in general relativity, an object falling towards a black hole would to an outside observer seem to have time pass infinitely slowly at the event horizon. The object itself would feel as if time flowed normally.

In order to reconcile quantum mechanics with black holes, Chapline theorized that a phase transition in the phase of space occurs at the event horizon. He based his ideas on the physics of superfluids. As a column of superfluid grows taller, at some point, density increases, slowing down the speed of sound, so that it approaches zero. However, at that point, quantum physics makes sound waves dissipate their energy into the superfluid, so that the zero sound speed condition is never encountered.

In the dark-energy star hypothesis, infalling matter approaching the event horizon decay into successively lighter particles. Nearing the event horizon, environmental effects accelerate proton decay. This may account for high energy cosmic ray sources and positron sources in the sky. When the matter falls through the event horizon, the energy equivalent of some or all of that matter is converted into dark energy. This negative pressure counteracts the mass the star gains, avoiding a singularity. The negative pressure also gives a very high number for the cosmological constant.

Furthermore, 'primordial' dark-energy stars could form by fluctuations of space-time itself, which is analogous to "blobs of liquid condensing spontaneously out of a cooling gas." This not only alters the understanding of black holes, but has the potential to explain the dark energy and dark matter, that are indirectly observed.