Category Archives: stellar ages

astronomy, astronomy education, astronomy news, astrophysics, fulldome, fulldome shows, galaxies, hubble space telescope, Loch Ness Productions, Mars, starbirth, stargazing, stellar ages

Across the Light-years from Andromeda

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Revisiting the Past

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Light Years from Andromeda, 2009

This week Mark and I are releasing a fulldome show version of the very first Loch Ness Productions planetarium show I ever wrote, called Light Years from Andromeda. It tells one of the most important stories in astronomy and cosmology — that of cosmic distance, and humanity’s quest to understand the universe. It is, as we say on the show’s Web page touting the show, “a journey of epic proportions across space and time.”

It seems particularly appropriate to talk about it today, with the release of a new, more precise value for the Hubble Constant. That constant is one of the numbers astronomers use to determine the expansion rate of the universe.  Knowing that expansion rate helps us also determine other factors like the size of the universe and just how old the universe really is. But, the root of all this knowledge is cosmic distance.

Distance in the universe is important to understand, and it is measured by using “standard candles” in the darkness. The standard candles are usually specific types of supernova explosions and, in particular,  a type of pulsating star called a Cepheid variable.  These pulsate with a regular rhythm and they are found in every galaxy we’ve seen so far. You can use the observations of those candles to derive distances across the cosmos.

In Light Years from Andromeda, we focus on a distance that most people have heard of, even if they aren’t up on the latest in cosmology: the light-year. It’s the distance that light travels in a year at a speed of roughly 300,000 kilometers per second. We wanted to bring that figure into some kind of reality for people, so I decided to take them out to the Andromeda Galaxy, which lies some 2.5 million light-years away.  We begin the show there, some 2.5 million years ago, and we bring people back home to the Milky Way, across all that time and space at the speed of light. Along the way we discuss human history set against that travel time. And, when we arrive at Earth, we learn about light-speed and the way that we can use light to measure cosmic distances.  It’s really a simple concept and a simple story to tell. And, it’s been very satisfying to see the show come to life in the new fulldome medium, where we really CAN fly from one galaxy to another and take in the breathtaking beauty of the cosmos.  And, to have it come out now, when the precision of the Hubble Constant is even better than before — well, it just sends chills up my spine. These are the times that make my day as a science writer!

(Check out the trailer for the show below.)

Light Years from Andromeda trailer

binary stars, blue stragglers, stars, stellar ages, stellar classification, stellar evolution

Blue Stragglers Explained

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Stellar Cannibalism the Mechanism that Creates Them

The core of globular cluster 47 Tucanae is home to many blue stragglers, rejuvenated stars that glow with the blue light of young stars. Courtesy NASA/STScI.
The core of globular cluster 47 Tucanae is home to many blue stragglers, rejuvenated stars that glow with the blue light of young stars. Courtesy NASA/STScI.

The astronomy zoo abounds with exotic-sounding creatures like magnetars and pulsars and brown dwarfs and hot Jupiters and other intriguingly named objects.  Today, astronomers are talking about another such denizen of the zoo–the blue straggler. These are massive stars that are found in globular clusters. These “overweight” objects have been seen throughout the universe and are stars that should have died a long time ago.

Theory says that most stars in a globular are formed around the same time and should die about the same time. Yet, these big, bloated bad boys hang on and on long after their crêche-mates have exploded and died.

So, what gives with these guys?

Researchers using data taken using Hubble Space Telescope have looked at blue stragglers in a number of clusters. They discovered that these mysterious overweight stars are actually the result of “stellar cannibalism.” Essentially, they keep living by eating up the plasma from nearby stars.  The result is a massive, unusually hot star that appears younger than it is (i.e. blue and hot).

Now, you need a binary star (two stars orbiting a common center of mass) to form a blue straggler for this process to work, and when scientists looked at the mechanics of how one star in a binary can gobble up its companion’s mass, it was the key that helped resolve the mystery of how blue stragglers come into existence.

How did the astronomers figure this out?

Two researchers from Canada — Dr. Christian Knigge  of Southampton University and Professor Alison Stills of McMaster University (both in Canada) looked at blue stragglers in 56 globular clusters to figure out how these stars –which astronomers have known about for 55 years — could form.

Sills pointed out that,  “Over time two main theories have emerged: that blue stragglers were created through collisions with other stars; or that one star in a binary system was “reborn” by pulling matter off its companion. “

In the course of looking at a collection of blue stragglers, the two researchers found that the total number of of these stars in a given cluster did not correlate with a predicted collision rate, which blew away a ­ theory that blue stragglers are created through collisions with other stars.

They did, however, discover a connection between the total mass contained in the core of a globular cluster and the number of blue stragglers observed within it. Since clusters with more massive cores also contain more binary stars, the astronomers were able to infer a relationship between blue stragglers and binaries in globular clusters. Preliminary observations directly measured the abundance of binary stars in cluster cores — and what they found supported the relationship.  All of this points to “stellar cannibalism” as the primary mechanism for blue straggler formation.

Knigge commented that the only thing that made sense for the creation of blue stragglers was that at least two stars had to be involved in the creation of every blue straggler.  That makes sense, since isolated stars that contained as much mass as typical blue stragglers do could not exist in clusters. “This is the strongest and most direct evidence to date that most blue stragglers, even those found in the cluster cores, are the offspring of two binary stars,” he said.

The two scientists are still intrigued by the mechanism for blue straggler formation and want to nail down the exact details. “We will want to determine whether the binary parents of blue stragglers evolve

mostly in isolation, or whether dynamical encounters with other stars in the clusters are required somewhere along the line in order to explain our results,” Knigge said.

If you’d like more details about the detailed analysis of blue stragglers in globulars, check out the 15 January edition of Nature Magazine (if you don’t have a subscription, check out your library for it).