Posted on by Fraser Cain

Why are Distant Galaxies Moving Away Faster?

Question: Why are more distant galaxies moving away faster?

Answer: As you know, the Universe is expanding after the Big Bang. That means that every part of the Universe was once crammed into a tiny spot smaller than a grain of sand. Then it began expanding, and here we are, 13.7 billion years later with a growing Universe.

The expansive force of dark energy is actually accelerating the expansion even faster. But we won’t bring that in to make things even more complex.

As we look out into the Universe, we see galaxies moving away from us faster and faster. The more distant a galaxy is, the more quickly it’s moving away.

To understand why this is happening, go and get a balloon (or blow one up in your mind). Once you’ve got it blown up a little, draw a bunch of dots on the surface of the balloon; some close and others much further away. Then blow up the balloon more and watch how the dots expand away from each other.

From the perspective of any one dot on the surface of the balloon, the nearby dots aren’t expanding away too quickly, maybe just a few centimeters. But the dots on the other side of the balloon are quite far away. It took the same amount of time for all the dots to change their positions, so the more distant dots appeared to be moving faster.

That’s how it works with the Universe. Because space itself is expanding, the more further a galaxy is, the faster it seems to be receding.

Thanks to Cassandra for the question.

Posted on by Fraser Cain

How Can Galaxies Recede Faster than the Speed of Light?

Question: How Can Galaxies Move Away Faster Than Speed of Light?

Answer: Einstein’s Theory of Relativity says that the speed of light – 300,000 km/s – is the maximum speed that anything can travel in the Universe. It requires more and more energy to approach the speed of light. You could use up all the energy in the Universe and still not be traveling at light speed.

As you know, most of the galaxies in the Universe are expanding away from us because of the Big Bang, and the subsequent effects of dark energy, which is providing an additional accelerating force on the expansion of the Universe.

Galaxies, like our own Milky Way are carried along by the expansion of the Universe, and will move apart from every other galaxy, unless they’re close enough to hold together with gravity.

As you look at galaxies further and further away, they appear to be moving faster and faster away from us. And it is possible that they could eventually appear to be moving away from us faster than light. At that point, light leaving the distant galaxy would never reach us.

When that happens, the distant galaxy would just fade away as the last of the photons reached Earth, and then we would never know it was ever there.

This sounds like it breaks Einstein’s theories, but it doesn’t. The galaxies themselves aren’t actually moving very quickly through space, it’s the space itself which is expanding away, and the galaxy is being carried along with it. As long as the galaxy doesn’t try to move quickly through space, no physical laws are broken.

One sad side effect of this expansion is that most of the galaxies will have receded over this horizon in about 3 trillion years, and future cosmologists will never know there’s a great big Universe out there.

You can read more about this in an article I did called the End of Everything.

Posted on by Fraser Cain

Space Questions for the Week Ending April 21st, 2008

Here is a round up of all the space questions answered on Universe Today for the week ending April 21st, 2008. If you have questions for our team, visit this page.

How Big Do Planets Get?
What is the Distance to the Moon?
What is the Far Side of the Moon?
What Kind of Star is Our Sun?
How Big is Apophis?
Why Do Stars Die?
Why are there Black Holes in the Middle of Galaxies?
Why Do Some Scientists Consider Pluto to Not Be a Planet?
Why Haven’t Planets Been Detected Around Alpha Centauri?
Why Can’t We Land on Mars?

Posted on by Fraser Cain

Space Questions Answered

As you can probably imagine, I get an enormous amount of email, both through Universe Today and Astronomy Cast. I read it all, and I try to answer it all – so far, so good; although, I’m a little behind right now.

Instead of just emailing people back answers to their questions, I figured the information would be helpful to many of you. So I’m writing up little mini-articles to answer the questions, and I’m filing them under a special Questions section of the website.

The problem is that I’m going a lot of these, and I’d like to do even more (two birds, one stone). But I don’t want to overwhelm the main page of the site and the RSS feed. I think it would dilute the news focus of Universe Today.

So I’ve got a listing of all the latest questions over on the right-hand side of the page, and they’re in their own Questions section. And I’ll probably provide a summary list of questions once a week in the main feed so you can read any that interest you.

If you’ve got a better idea… I’m all ears. I’ll follow this story with an example of a summary.

If you’ve got a question, here’s where to go.

Posted on by Fraser Cain

How Big Do Planets Get?

Artist's impression of Gliese 436 c

Question: How Big Can Planets Get?

Answer: Here in the Solar System, we have three kinds of planets: the inner terrestrial planets, the gas giants, and the ice planets. Sadly, Pluto is no longer a planet, so we won’t deal with that here. We know how big our planets are, but how big can planets actually get in other Solar Systems. What are the biggest possible planets?

Let’s start with terrestrial planets, like our Earth. We’ll set the size of the Earth and 1 Earth radius, and the mass as 1 Earth mass. We’ve seen that terrestrial planets can get smaller, with Mars and Mercury, and astronomers have detected larger terrestrial planets orbiting other stars.

The largest known rocky planet is thought to be Gliese 436 c. This is probably a rocky world with about 5 Earth masses and 1.5 times our planet’s radius. Amazingly, this planet is thought to be within its star’s habitable zone.

What’s the largest possible rocky planet? For this I put in an email to Dr. Sean Raymond, a post doctoral researcher at the Center for Astrophysics and Space Astronomy (CASA) at the University of Colorado. Here’s what he had to say:

“The largest “terrestrial” planet is generally considered the one before you get too thick of an atmosphere, which happens at about 5-10 Earth masses (something like 2 Earth radii). Those planets are more Earth-like than Neptune-like.”

Gas giants, of course, can come much larger. Jupiter is 317 times more massive than Earth, and 11 times larger. You could fit 1,400 Earths inside Jupiter.

Thebiggest planet in the Universe (at the time of this writing) is TrES-4, which is located 1,400 light years away in the constellation Hercules. The planet has been measured to be 1.4 times the size of Jupiter, but it only has 0.84 times Jupiter’s mass. With such a low density, the media was calling TrES-4 the puffy planet.

And once again, how large can they get? Again, here’s Dr. Raymond:

“In terms of gaseous planets, once they reach 15 Jupiter masses or so there is enough pressure in the core to ignite deuterium fusion, so those are considered “brown dwarfs” rather than planets.”

What is the biggest planet in the Solar System?

Posted on by Fraser Cain

How Big is Apophis?

Radar image of asteroid Apophis.

Question: How Big is Apophis?

Answer: In case you haven’t heard, Asteroid 99942 Apophis is a near Earth asteroid that astronomers think will make a close flyby to the Earth in 2029. When its trajectory was first calculated back in 2004, it had one of the closest visits to Earth astronomers had seen, and had a 2.7% chance of hitting the Earth.

But follow-up observations brought that risk down to 1 in 45,000. Right now, astronomers think that Apophis is essentially no risk to the Earth. In April, 2008 media reported that a 13-year old German student had caught a math mistake made by NASA, and the risk of an Earth strike was actually 1-45. This later turned out to be a hoax.

Because of its close approach to Earth, space advocacy societies, including the Planetary Society think that the Apophis asteroid would make an ideal target for a human mission, and allow engineers to test out strategies for moving asteroids away from dangerous Earth-crossing orbits.

So back to the original question, how big is Apophis? The best estimate puts it at 270 meters (885 feet across), and it has a mass of 2.1 x 1010 kg. To give you a sense of scale, the Eiffel Tower in Paris is 324 meters tall.

But now you know its mass and size, you’re probably wondering: what would happen to the Earth if it struck? NASA estimated that a strike by Apophis would release the equivalent of 880 megatons of energy. Just as a comparison, the object that carved out Meteor Crater in Arizona probably released 3-10 megatons of energy.

If Apophis struck land, it would flatten thousands of square km of land, killing millions of people if it hit a densely populated area. But it wouldn’t cause the kinds of long term climate destruction that 1 km and larger asteroids can do. If it hit an ocean, it would create devastating tsunamis in all directions.

Here’s an article explaining techniques that might be used to move an asteroid. And here’s NASA’s official page on Apophis.

Posted on by Fraser Cain

Why are there Black Holes in the Middle of Galaxies?

Question: Why are Black Holes in the Middle of Galaxies?

Answer: The black holes you’re thinking of are known as supermassive black holes. Stellar mass black holes are created when a star at least 5 times larger than the Suns out of fuel and collapses in on itself forming a black hole. The supermassive black holes, on the other hand, can contain hundreds of millions of times the mass of a star like our Sun.

Astronomers are now fairly certain that these supermassive black holes are at the heart of almost every galaxy in the Universe. Furthermore, the mass of these black holes is somehow tied to the mass of the rest of the galaxy. They grown in tandem with each other.

When large quantities of material falls into the black hole, it chokes up, unable to get consumed all at once. This “accretion disk” begins to heat up and blaze brightly in many different wavelengths, including X-rays. When supermassive black holes are actively feeding, astronomers call these quasars.

So how do these black holes get there in the first place? Astronomers aren’t sure, but it could be that the dark matter halo that surrounds every galaxy serves to focus and concentrate material as the galaxy was first forming. Some of this material became the supermassive black hole, while the rest became the stars of the galaxy. It’s also possible that the black hole formed first, and collected the rest of the galaxy around it.

Astronomers just don’t know.

Posted on by Fraser Cain

Why Haven’t Planets Been Detected Around Alpha Centauri?

Toliman
Artist impression of Alpha Centauri

Question: Why aren’t astronomers looking for planets around nearby stars like Alpha Centauri?
Answer: That’s a great question. Since Alpha Centauri is only a little over 4 light-years away, why aren’t astronomers studying it for planets, instead of the more distant stars.

Astronomers have included stars like Alpha Centauri in their search for extrasolar planets, they just haven’t found them yet. That’s because the techniques used to find extra solar planets require very large planets orbiting very close to their parent stars.

The first technique is called the radial velocity method. This is where the gravity of the planet yanks its parent star back and forth. The changes in the star’s velocity are measurable in the light that reaches the Earth.

The second technique looks for transits. This is where the planet passes in front of the parent star, dimming it slightly. By measuring the amount the light dims, astronomers are able to know if there’s a planet there, calculate its size and even determine what’s in its atmosphere.

A third technique detects microlensing events. A closer star focuses the light from a more distant star with its gravity. From Earth, we see a flare in brightness as the two stars line up perfectly. If the closer star has a planet orbiting it, that will change the light curve that astronomers detect, allowing them to calculate the size of the planet.

Most of the planets discovered to date are known as Hot Jupiters. These are planets much larger than Jupiter that orbit within the orbit of Mercury.

A team of astronomers led by Javiera Guedes from the University of California think that an Earth-sized planet should be detectable orbiting Alpha Centauri. They’re working to get a single dedicated telescope to watch the star, and work out if there are planets there. According to their calculations, it should only take about 5 years of intense observations by a dedicated telescope to work out the answer.

Posted on by Fraser Cain

Stuart Blasts Star Naming Companies

When I pitched a story idea to Tammy Plotner about how a president of a public observatory deals with those companies that offer to name stars, I was expecting (hoping) for something pretty negative. I think my request was something like, “could you debunk this please”? The article she actually wrote, Name a Star – Real or Ripoff, was much more nuanced, honest and forthright than I was expecting – I’m really glad she took the direction she did, and reminds me why I’m glad Tammy’s writing for Universe Today.

It’s an interesting dilemma. These people are selling something they have no right to sell and have to be dishonest about it. The best companies provide the coordinates to a real star, and the worst…

Anyway, Stuart over at Cumbrian Sky wrote the opinion I was expecting from Tammy. Completely hard line, but a very well defended position. It’s a great read – Star Naming, Fun or Foul.

If any other blogs out there want to chime in on this topic, I’ll be happy to update this page and link to your opinions as well.

Posted on by Fraser Cain

A Portal to Another Universe?

In episode 56 of Astronomy Cast, I noted that hoping that a black hole will lead to another dimension is sort of like a frog thinking that a blender will take him to another realm.

Astronomy Cast listener Isaac Windham animated the sequence, just to really drive the point home…

And here’s the transcript from the show, so you’ll all get the reference. Thanks Isaac!

Fraser: Why do people think we might live in a black hole? That seems kind of crazy to me.

Pamela: It’s a lot of science fiction. There’s this idea in science fiction that you can fly into a black hole and emerge in a completely different part of our universe, in an alternate universe… and so from these fiction writings, the idea has gotten into the zeitgeist that you fly into a black hole and you fly into a different universe – which means a universe can be inside of a black hole.

The problem is real black holes just lead to death.

Fraser: I guess that’s the question – it’s like a frog asking if I hop into that blender, will it lead me to another universe?

Pamela: Exactly

Fraser: No, no it won’t – a universe of pain.

Pamela: It will lead to death, and yeah – where death leads to is a personal question not based in facts and not addressable in this show.

Fraser: Right, so it’s almost like it’s become a kind of philosophical question and it goes back to that extra-dimensional conversation we had in a well-received episode we did back in the day. I guess it’s kind of like it’s different – could it be so different that it’s not really a devastating matter crusher? Could it be a bold new universe we could explore? (Says the frog hopping into his blender.