Last call for the reader survey!

http://bit.ly/mysciblogreaders

Today’s the last day to fill in the reader survey we’ve been running in collaboration with Paige Brown Jarreau, so if you’ve been putting it off, midnight Central time is your deadline!

If you haven’t thought about filling in the survey, please do! If you have read even one post, you can do it!

It should take ~10 minutes, and everyone gets science art from Paige’s Photography as a thank you for filling in the survey.

You can also be entered into a drawing for $50 gift cards! At least two Astroquizzical readers will definitely win (there are 100 gift cards total), so if you’d like a shot at the prize while also helping me tell you about science, please click below and complete the survey! Many thanks!

http://bit.ly/mysciblogreaders

Reader survey!

Who are you, my readers?

Hi tumblr! Astroquizzical is participating in a first-of-its-kind scientific survey of the people who read science blogs, and why! But the survey needs your participation!

If you have read any Astroquizzical posts (ever), you can participate! Click on the link here: http://bit.ly/mysciblogreaders !

At least 2 of you will definitely get a $50 prize to thank you for your time, and there are art prizes for everyone who completes the survey.  (All your information will be held anonymously, so I won’t wind up with your names etc.)

If you have reblogged Astroquizzical before, please reblog this post! I’d like everyone who’s seen a post of mine to have a chance to fill in the survey.  Thanks!

http://bit.ly/mysciblogreaders

-your friendly neighborhood astrophysicist

(via astroquizzical)

EDIT: If you’re having trouble with the bitly link, the direct link is here:

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Why doesn't a globular cluster collapse?

Why doesn’t a globular star cluster collapse into a single body due to the mutual gravitational effects of the individual stars?
This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Image credit:  The Hubble Heritage Team (AURA/STScI/NASA)

This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Image credit: The Hubble Heritage Team (AURA/STScI/NASA)

Originally posted @ Medium!

Globular clusters are strange objects. They’re incredibly old collections of a huge number of stars — somewhere around a few hundred thousand on average, and they orbit around the very edges of galaxies, dotted spherically around. Our own Milky Way has a few hundred that we’ve found — Andromeda, the nearest other large galaxy we can check, has at least 60 some. (We’re likely to miss some around Andromeda, since they’re not the brightest things, and are quite small.) Even some of the Milky Way’s tiny nearby satellite galaxies have their own collection of globular clusters.

When I say that these clusters are incredibly old, we’re talking about as old as you can get. Some of the stars in a globular cluster are so old that they can help us to constrain the age of the universe. (If your globular cluster is nearly 13 billion years old, you can’t have a Universe younger than that.) We’re still not quite sure what conditions are needed for a globular cluster to form, or how they manage to survive for so long.

This illustration shows the location of the globular cluster M4 in our Milky Way Galaxy, which is depicted 'edge-on' or from the side. Globular clusters like M4 are the first pioneer settlers of the Milky Way. Many coalesced to build the hub of our galaxy and formed billions of years before the appearance of the Milky Way's magnificent pinwheel disk. Today, 150 globular clusters survive in the galactic halo. Image Credit:  NASA  /  ESA   and A. Feild

This illustration shows the location of the globular cluster M4 in our Milky Way Galaxy, which is depicted 'edge-on' or from the side. Globular clusters like M4 are the first pioneer settlers of the Milky Way. Many coalesced to build the hub of our galaxy and formed billions of years before the appearance of the Milky Way's magnificent pinwheel disk. Today, 150 globular clusters survive in the galactic halo. Image Credit: NASA/ESA and A. Feild

One thing you can take from their age is that they’re relatively stable. Which, as you have guessed, means that they can’t be in the process of collapsing down onto themselves, or we wouldn’t see so many of them looking so similar. So how do they manage to stay the same size over time?

At a very basic level, you might expect that if you suspended 100,000 stars in space, and let time roll forward, gravity would simply pull everything together. Some stars might get twirled around for a time, but gravity would ultimately crush everything into a mess at the very center of what used to be a globular cluster. And that’s exactly what would happen, if gravity were the only thing at play.

This NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Image credit:  ESA/Hubble & NASA

This NASA/ESA Hubble Space Telescope image shows a compact and distant globular star cluster that lies in one of the smallest constellations in the night sky, Delphinus (The Dolphin). Image credit: ESA/Hubble & NASA

But the stars are moving. And when you have motion, you can defeat gravity for a time. Each star is in orbit around the center of the globular cluster, and the motion of the star in a sideways fashion, combined with the pull of gravity, creates an oval-shaped orbit, and the star can circle the center of the globular cluster reasonably happily for quite some time. Because the star has enough energy to keep moving forward, the tug of gravity won’t be able to haul the star to the center of the cluster.

This is a bit of a simplified picture, because every single other star of the 100,000+ in the cluster is doing the exact same thing, and the orbits of the stars are effectively random, which is why the cluster looks so spherical. If the orbits of the stars were less perfectly scrambled, you would see the cluster look a little more elongated, like a disk (like a spiral galaxy does, from the side), instead of looking pretty much round. Elliptical galaxies, for instance, also have done a pretty effective job of scrambling the orbits of the stars within them, so they look pretty round as well — but contain many millions more stars than a globular cluster. If the stars of an object are in this sort of randomized configuration (instead of rotating nice and orderly in a disk) we call it pressure supported (as opposed to rotation supported). In this context, it’s not that there’s actually any physical push outwards that makes them pressure supported, but the random motions of the stars as they each orbit their center of gravity acts as a resisting force to gravity. It’s not pressure, but it’s not ordered rotation, and it still resists gravity, so: pressure. (Astronomers are bad at naming things.)

The object shown in this beautiful Hubble image, dubbed Messier 54, could be just another globular cluster, but this dense and faint group of stars was in fact the first globular cluster found that is outside our galaxy.  Image credit:  ESA/Hubble & NASA

The object shown in this beautiful Hubble image, dubbed Messier 54, could be just another globular cluster, but this dense and faint group of stars was in fact the first globular cluster found that is outside our galaxy.  Image credit: ESA/Hubble & NASA

Just because these systems are reasonably old and reasonably stable doesn’t mean they don’t change over time, and one thing that does occur with time is that the stars within the cluster will get close enough to each other to interact gravitationally. Not all the stars in the cluster are the same size, so the way they interact will depend on their mass. If the two stars are (for simplicity) roughly equal in mass, but one happens to be moving a bit faster, then the faster star will donate some of its energy to the slower star, giving them both a similar speed when they leave each other’s company, under most circumstances. (There’s no requirement for them to be going the same direction.) If, however, the stars are very different in mass, and they split their total energy, the less massive star will wind up going much faster. Kinetic energy is equal to the mass times the velocity squared — a large mass means a much smaller velocity for the same amount of energy, and vice versa.

This means that over time, the massive stars will tend to slow down, and the light stars will tend to speed up. If a star is slowing down, then gravity gets to take over, and does indeed pull that star down, closer to the core of the cluster. You wind up with some globular clusters collecting all their high mass stars in their very centers, with the lightest stars zooming around the outskirts, going much faster. This pattern of energy allows the cluster to sort itself from heaviest to lightest. (The technical term for this end result is called ‘mass segregation’.)

This image shows a globular cluster known as NGC 104 — or, more commonly, 47 Tucanae, since it is part of the constellation of Tucana (The Toucan) in the southern sky. After Omega Centauri it is the brightest globular cluster in the night sky, hosting tens of thousands of stars. Image credit:  ESA/Hubble & NASA

This image shows a globular cluster known as NGC 104 — or, more commonly, 47 Tucanae, since it is part of the constellation of Tucana (The Toucan) in the southern sky. After Omega Centauri it is the brightest globular cluster in the night sky, hosting tens of thousands of stars. Image credit: ESA/Hubble & NASA

The other thing that can cause change in a globular cluster is if its orbit takes it too close to the massive galaxy it sits near. Each globular cluster is, as a whole, orbiting a fairly massive galaxy, particularly in comparison to the cluster. This orbit takes a very long time to complete, and it seems that many globular clusters can successfully orbit the galaxy without getting too near — but if the cluster does get too near, then the cluster will experience an intense tidal force because of the gravitational pull of the galaxy. This tidal force can shear off the outer layers of the cluster (if it’s a mass-segregated cluster, this means it looses the smallest stars). These outer layers get pulled away into a long stream of stars, barely detectable in the night sky even with a powerful telescope, and leaving an even denser nucleus of stars behind; at least four such globulars exist in the interior of our own Milky Way.

Although globular clusters don’t collapse, it’s conceivable that the most massive stars — which give rise to black holes — will segregate the fastest, giving rise to an intermediate mass black hole at the centers of globulars. On the other hand, it’s conceivable that black holes are ejected too frequently, as massive stars tend to give rise to far less massive black holes. While stellar-mass black holes have been observed inside a few globular clusters, there’s no consensus as to whether they can contain larger ones at their centers or not; that’s still an open question!



Participate in the reader survey! Do a science, maybe get $50!

http://bit.ly/mysciblogreaders

Help me do science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of Astroquizzical readers. By participating, you’ll be helping me improve Astroquizzical and you’ll be contributing to SCIENCE (on blog readership).

You will get FREE science art from Paige’s Photography for participating, as well as a chance to win $50! (At least two Astroquizzical readers will definitely get $50, but there are 100 $50 prizes available.) There are also t-shirts and other perks! It should only take 10-15 minutes to complete. You can find the survey here: http://bit.ly/mysciblogreaders.

In the future, will we need more time to travel between galaxies?

Each galaxy is expanding away from the others. As a result, would that mean we require more time in the future to travel to other galaxies as the space in between increases as well?
A computer model shows one scenario for how light is spread through the early universe on vast scales (more than 50 million light years across) Image credit:  Andrew Pontzen/Fabio Governato

A computer model shows one scenario for how light is spread through the early universe on vast scales (more than 50 million light years across) Image credit: Andrew Pontzen/Fabio Governato

Originally posted at Forbes!

It depends on how far you want to go! Generally, when an astronomer is trying to explain the idea of space expanding, they might use the idea of the space between two galaxies expanding. This is a good illustration; it communicates the idea that it’s not that the galaxies are in motion themselves, but that the space they’re embedded within is growing, so the distance between the two of them wind up growing with time.

Unfortunately, this explanation can have a bit of an unintended side effect as a metaphor- if I’m not careful, I can wind up simplifying the universe too much, so that it seems like this happens equally to all galaxies, with galaxies spaced out roughly equally from each other across the universe. But if we actually look out into the night sky, we can see that ours is not a universe of equal spacing. Ours is a universe that looks rather spidery, with filaments of the cosmic web stretching across the sky. The image at the top shows a simulation of the distribution of galaxies in our universe. There are grand areas of nothing, and clusters of thousands of galaxies, connected to other clusters with faint tendrils, each made up of glowing galaxies.

And yet, the universe is expanding, and the space between objects is increasing, due to the currently inexplicable influence of Dark Energy. Should this affect, say, the distance between the Milky Way and Andromeda?

In principle it might – but it’s important to remember two facets to the universe’s expansion. One – this expanding force is very, very small. It happens that we have an awful lot of space, so over the whole universe it’s quite significant. The second thing is that on reasonably small scales (which in this case, means objects which are the size of galaxies and smaller), gravity is much stronger.

Gravity is a bit of a juggernaut in the astrophysical world, and if two objects are gravitationally bound to each other (meaning that their relative speeds are too slow to let them escape the pull of the gravity of the other), cosmic expansion isn’t going to be able to do much about it. The expansion of the universe would have to be frighteningly enormous (much larger than we observe it to be) to pull apart two galaxies which are bound to each other by gravity. The Milky Way and Andromeda are bound, and as such, are due to collide sometime in the next 3.5 – 4.5 billion years. So if you want to go to Andromeda at any point in the future (before it collides with us), cosmic expansion won’t play a role in the time it would take to travel there. It’s not just Andromeda that we’re bound to – we have a whole cluster of galaxies that our galaxy is part of, and all of these are bound to each other gravitationally, though much more loosely than the Milky Way is to Andromeda.

The location of the Milky Way with respect to the other galaxies within our Local Group. Image credit  Andrew Z. Colvin, CC 3.0 BY A-SA 3.0

The location of the Milky Way with respect to the other galaxies within our Local Group. Image credit Andrew Z. Colvin, CC 3.0 BY A-SA 3.0

But if you wanted to go beyond our cluster of galaxies, and travel intergalactic space to reach a totally separate cluster, one entirely unrelated to our galaxy – then, yes, you would need more time to travel to those galaxies in the future than you would if you left today.

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BONUS ANNOUNCEMENT!

Help me do science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of Astroquizzical readers. By participating, you’ll be helping me improve Astroquizzical and you’ll be contributing to SCIENCE (on blog readership).

You will get FREE science art from Paige’s Photography for participating, as well as a chance to win $50! (At least two Astroquizzical readers will definitely get $50, but there are 100 $50 prizes available.) There are also t-shirts and other perks! It should only take 10-15 minutes to complete. You can find the survey here: http://bit.ly/mysciblogreaders.

Go science it up, space-curious Astroquizzical readers!

But if you wanted to go beyond our cluster of galaxies, and travel intergalactic space to reach a totally separate cluster, one entirely unrelated to our galaxy – then, yes, you would need more time to travel to those galaxies in the future than you would if you left today.



BONUS ANNOUNCEMENT!

Help me do science! I’ve teamed up with researcher Paige Brown Jarreau to create a survey of Astroquizzical readers. By participating, you’ll be helping me improve Astroquizzical and you’ll be contributing to SCIENCE (on blog readership).

You will get FREE science art from Paige’s Photography for participating, as well as a chance to win $50! (At least two Astroquizzical readers will definitely get $50, but there are 100 $50 prizes available.) There are also t-shirts and other perks! It should only take 10-15 minutes to complete. You can find the survey here: http://bit.ly/mysciblogreaders.

Go science it up, space-curious Astroquizzical readers!