Weekly Space Hangout – October 14, 2016: Sally Ride EarthKAM project

Host: Fraser Cain (@fcain) Special Guest: Guests: Tyler Finlay of the Sally Ride EarthKAM project Guests: Morgan Rehnberg (MorganRehnberg.com / @MorganRehnberg) Their stories this week: Two trillion galaxies? Obama reaffirms NASA’s Mars plan ExoMars arrives in the coming week CosmoQuest Survey for Citizen Scientists – how can we make citizen science more available? CosmoQuest Survey […]

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The Big Dipper in the Year 92,000

Stellar motions distort the future sky. Map: Bob King, Source: Stellarium

You go out and look at the stars year after year and never see any of them get up and walk away from their constellations. Take a time machine back to the days of Plato and Socrates and only careful viewing would reveal that just three of the sky’s naked eye stars had budged: Arcturus, Sirius and Aldebaran. And then only a little. Their motion was discovered by Edmund Halley in 1718 when he compared the stars’ positions then to their positions noted by the ancient Greek astronomers. In all three cases, the stars had moved “above a half a degree more Southerly at this time than the Antients reckoned them.”

Stars are incredibly far away. I could throw light years around like I often do here, but the fact is, you can get a real feel for their distance by noting that during your lifetime, none will appear to move individually. The gems of the night and our sun alike revolve around the center of the galaxy. At our solar system’s distance from the center — 26,000 light years or about halfway from center to edge — it takes the sun about 225 million years to make one revolution around the Milky Way.

That’s a LONG time. The other stars we see on a September night take a similar length of time to orbit. Now divide the average lifetime of some 85 years into that number, and you’ll discover that an average star moves something like .00000038% of its orbit around the galactic center every generation. Phew, that ain’t much! No wonder most stars don’t budge in our lifetime.

Sirius, Aldebaran and Arcturus and several other telescopic stars are close enough that their motion across the sky becomes apparent within the span of recorded history. More powerful telescopes, which expand the scale of the sky, can see a great many stars amble within a human lifetime. Sadly, our eyes alone only work at low power!

But we needn’t invest billions in building a time machine to zing to the future or past to see how the constellation outlines become distorted by the individual motions of the stars that compose them. We already have one! Just fire up a free sky charting software program like Stellarium and advance the clock. Like most such programs, it defaults to the present, but let’s look ahead. Far ahead.

If we advance 90,000 years into the future, many of the constellations would be unrecognizable. Not only that, but more locally, the precession of Earth’s axis causes the polestar to shift. In 2016, Polaris in the Little Dipper stands at the northernmost point in the sky, but in 90,000 years the brilliant star Vega will occupy the spot. Tugs from the sun and moon on Earth’s equatorial bulge cause its axis to gyrate in a circle over a period of about 26,000 years. Wherever the axis points defines the polestar.

Take a look at the Big Dipper. Wow! It’s totally bent out of shape yet still recognizable. The Pointer Stars no longer quite point to Polaris, but with some fudging we might make it work. Vega stands near the pole, and being much closer to us than the rest of Lyra’s stars, has moved considerably farther north, stretching the outline of the constellation as if taffy.

Time goes on. We look up at the night sky in the present moment, but so much came before us and much will come after. Constellations were unrecognizable in the past and will be again in the future. In a fascinating discussion with Michael Kauper of the Minnesota Astronomical Society at a recent star party, he described the amount of space in and between galaxies as so enormous that “we’re almost not here” in comparison. I would add that time is so vast we’re likewise almost not present. Make the most of the moment.

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Best Picture Yet Of Milky Way’s Formation 13.5 Billion Years Ago

The Milky Way is like NGC 4594 (pictured), a disc shaped spiral galaxy with around 200 billion stars. The three main features are the central bulge, the disk, and the halo. Credit: ESO

Maybe we take our beloved Milky Way galaxy for granted. As far as humanity is concerned, it’s always been here. But how did it form? What is its history?

Our Milky Way galaxy has three recognized stellar components. They are the central bulge, the disk , and the halo. How these three were formed and how they evolved are prominent, fundamental questions in astronomy. Now, a team of researchers have used the unique property of a certain type of star to help answer these fundamental questions.

The type of star in question is called the blue horizontal-branch star (BHB star), and it produces different colors depending on its age. It’s the only type of star to do that. The researchers, from the University of Notre Dame, used this property of BHB’s to create a detailed chronographic (time) map of the Milky Way’s formation.

This map has confirmed what theories and models have predicted for some time: the Milky Way galaxy formed through mergers and accretions of small haloes of gas and dust. Furthermore, the oldest stars in our galaxy are at the center, and younger stars and galaxies joined the Milky Way over billions of years, drawn in by the galaxy’s growing gravitational pull.

The team who produced this study includes astrophysicist Daniela Carollo, research assistant professor in the Department of Physics at the University of Notre Dame, and Timothy Beers, Notre Dame Chair of Astrophysics. Research assistant professor Vinicius Placco, and other colleagues rounded out the team.

“We haven’t previously known much about the age of the most ancient component of the Milky Way, which is the Halo System,” Carollo said. “But now we have demonstrated conclusively for the first time that ancient stars are in the center of the galaxy and the younger stars are found at longer distances. This is another piece of information that we can use to understand the assembly process of the galaxy, and how galaxies in general formed.”

The Sloan Digital Sky Survey (SDSS) played a key role in these findings. The team used data from the SDSS to identify over 130,000 BHB’s. Since these stars literally “show their age”, mapping them throughout the Milky Way produced a chronographic map which clearly shows the oldest stars near the center of the galaxy, and youngest stars further away.

“The colors, when the stars are at that stage of their evolution, are directly related to the amount of time that star has been alive, so we can estimate the age,” Beers said. “Once you have a map, then you can determine which stars came in first and the ages of those portions of the galaxy. We can now actually visualize how our galaxy was built up and inspect the stellar debris from some of the other small galaxies being destroyed by their interaction with ours during its assembly.”

Astronomers infer, from various data-driven approaches, that different structural parts of the galaxy have different ages. They’ve assigned ages to different parts of the galaxy, like the bulge. That makes sense, since everything can’t be the same age. Not in a galaxy that’s this old. But this map makes it even clearer.

As the authors say in their paper, “What has been missing, until only recently, is the ability to assign ages to individual stellar populations, so that the full chemo-dynamical history of the Milky Way can be assessed.”

This new map, with over 130,000 stars as data points, is a pretty important step in understanding the evolution of the Milky Way. It takes something that was based more on models and theory, however sound they were, and reinforces it with more constrained data.

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