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Bring him to me!

February 29, 2008 on 3:06 pm | In Astronomy, Gravity, Physics, Video | 2 Comments

The Milky Way galaxy is a relatively big spiral galaxy. So is Andromeda. There are about 20 dwarf galaxies that are gravitationally bound to us; combined with us, all of this makes up the local group. But Andromeda is moving towards us, and eventually, it’s going to merge with us. I’ll once again show you a video of what this merger might look like:

But what would we see, here in the Milky Way, as Andromeda got closer and closer to us? Right now, Andromeda looks like this:

But Andromeda is also very far away: about 2.3 million light years (770 kpc). The center of it is tiny on the sky, but the whole galaxy, as seen above, is actually 4 degrees across, or eight times larger than the diameter of the full moon! Its apparent magnitude is 4.4, which means it can barely be seen with the naked eye (anything less than 6 can be seen with your eye) if your vision is good and there’s no light pollution.

But the Universe will continue to age, and gravity will basically tell Andromeda “Get over here!” When this happens, Andromeda not only gets closer to us, but also starts to appear bigger and brighter in the sky. What does this mean? Let’s play Zeno’s Paradox, and see what happens when it gets halfway to us, and then halfway of that distance, etc.

  • About 1.9 billion years from now, Andromeda will be 385 kpc away from us. It now has an apparent magnitude of 2.9, which means it’s just barely visible from most urban neighborhoods, and appears slightly brighter than our own Milky Way does. It now takes up 8 degrees on the sky, making it 16 times as large as the Moon in diameter.
  • About 2.7 billion years from now, it will be within 190 kpc of us. That’s still well outside the Milky Way, which is less than 20 kpc in radius. It’s now quite bright, though, with an apparent magnitude of 1.4, making it as bright as one of the brightest stars in the sky, Regulus. It is now 16 degrees on the sky. If, at this point, it were oriented face-on to us, it would take up about 1% of the entire visible night sky.
  • About 3.2 billion years from now, it will be under 100 kpc from us. It now takes up 1/20 of the entire night sky, and only the Moon, the Planets, and three stars, Sirius, Canopus, and Alpha Centauri are brighter than Andromeda appears.
  • 3.4 billion years from now, Andromeda will be within 50 kpc of us, on the verge of beginning to merge with us. (Remember, it has a radius of about 20 kpc, too.) Its apparent magnitude is -1.5, meaning that it is now brighter than any star in the sky. It will now take up about one-fifth of the night sky, and will just begin to create new star-forming regions in the outskirts of the galaxies, where the gas begins to merge.

And then the merger happens. What will that do to us? Take a look:

Although we don’t know exactly what’s going to happen, it’s a good bet that we won’t want to be here for it. Time to find a new galaxy… or at least a temporary home outside of ours while that merger takes place!


Carnival of Space #43: Oscar Edition

February 27, 2008 on 9:36 pm | In Astronomy, Blog info, Scientific papers | 32 Comments

It’s been a spectacular week for the film space industry, and here at Starts With A Bang!, we’ve got the recap of all the highlights that you may have missed while watching the countless Oscar montages. Take your time browsing and enjoying this site, and maybe even find out what the question is if 42 is the answer! And now, without further ado, here are the winners from the 43rd Carnival of Space, as chosen by Ethan Siegel, your magnanimous host of this week’s Carnival (and check out all previous carnivals here):

Made it this far? Take a good look around the site and enjoy! Have any questions about astronomy/physics/cosmology? Drop me a line and check back; it’s your good questions that keep me writing. Thanks for visiting this week’s Carnival of Space! Did you like it? Digg it!


How old is the Sun in Galactic years?

February 27, 2008 on 2:23 pm | In Astronomy, Gravity, Physics, Q & A, Solar System | 18 Comments

The Moon goes around the Earth, the Earth goes around the Sun, and the Sun goes around the center of the Milky Way. We know the Moon takes about 4 weeks to make its trip around the Earth, and that causes the Moon phases:

We also know that the Earth takes one year to go around the Sun, and that causes the seasons:

We also know that the Earth has been around for about 4.5 billion years, which means it has gone around the Sun about 4.5 billion times. Well, now I ask the question(s):

How long does it take the Sun to go around the Milky Way? How many times has it done that so far, and how many times will this happen before the Sun finally dies?

Well, we know that we travel in (roughly) a circle around the center of the Milky Way, and that our radius from the center is about 8 kiloparsecs, or roughly 26,000 light years. That means our Solar System (including the Sun) needs to travel a distance of 1.55 x 1018 kilometers to go around the Milky Way once. If we know how fast the Sun is moving, we can figure out how long a Galactic Year is. Well, we can both measure and calculate its velocity to be 220 kilometers/second, and so we can just do the math, knowing that there are 31,556,952 seconds in a Gregorian Year, and we find that it takes about 223 million years to make one galactic year.

So, if the Sun is 4.5 billion years old, that makes it about 20 galactic years old. If the Sun has a total lifetime of around 10 billion years, then it has a total galactic age of around 42 galactic years.

What? Did I just say the answer is 42?! Well, this means that one possible question is “What is the Sun’s lifetime in Galactic Years?”


How did Life on Earth Originate?

February 26, 2008 on 3:20 pm | In Evolution, Life | 3 Comments

I was thinking about the timeline that brought us here, today, from the origin of the Universe up through the present day. I realized that the most uncertain thing that we know of, the step that we have the least information about, is the origin of life on Earth. All hypotheses about how life on Earth originated fall into three categories:

  1. Abiogenesis, or the idea that life came from non-life, somehow, on Earth.
  2. Life originated elsewhere in the Universe, and was brought to Earth, where it now thrives (e.g., panspermia, or exogenesis).
  3. Life was created or designed by an outside force/being.

Is that it? Does this really encompass everything? Well, I can’t think of another that doesn’t fall into one of these three categories. It should be noted that none of these three have been proven, and that work on a mechanism for abiogenesis, including Oparin’s hypothesis, the Miller-Urey experiments, and subsequent work has not yet yielded life generated from non-life.

However, the panspermia hypothesis has always bothered me, seeming like a cop-out. After all, can you really say the “origin of life” just happened somewhere else, and so we don’t care to investigate any further? Then I came across another one of cdk007’s youtube videos (from 2006) that articulates what those three options above are quite cleanly:

In a nutshell, alien life had to originate somewhere. If you’re asking about the origin of life in the Universe, the origin of the first living thing that we descended from, there are only two options:

  1. Either life originated from non-life naturally, or
  2. Life originated supernaturally.

If we can show that it’s possible for life to originate from non-life, that will remove the last remaining major gap in the scientific creation of our modern world, beginning from the big bang all the way through the present day. Of course, we don’t know how the whole thing started, or where it’s headed, but we’re always learning…


Q & A: Why is the Microwave Background so Uniform?

February 26, 2008 on 10:41 am | In Astronomy, Dark Energy, Physics, Q & A | 14 Comments

startswithabang.com reader Andy has a great follow-up to his question on the Age & Size of the Universe, and asks:

why does the CBR “appear” to come from a light sphere that “appears” NOW to be larger than the universe WAS when it first set off in a straight line on its 13.4 billion year trip???

The “CBR” stands for “Cosmic Background Radiation,” and it refers to the (presently) microwave background. Here’s why Andy’s question is actually profound, and was known for about 20 years as either the homogeneity problem or the horizon problem. The problem is that, when we look up at the sky, and take a look at the microwave radiation (that’s the leftover radiation from the big bang), we find that it’s the same temperature, 2.725 Kelvin, in every direction (top image at left). Arno Penzias and Robert Wilson won the Nobel Prize for discovering this uniform microwave background.

How uniform is it? If we subtract off 2.725 Kelvin from the entire image, we can measure the deviations from the mean temperature. What we find is that there is a range from -0.004 (blue) to +0.004 (red) Kelvin deviating from that mean temperature (middle image at left). What causes that? The motion of us against the microwave background, or a doppler shift from our local velocity. The discovery of this was also worth a Nobel Prize, this time for George Smoot and John Mather, who co-discovered it.

More recently, we’ve discovered that if you take away that doppler shift as well, you still find hot and cold spots, but these are only about 0.00003 Kelvin in range from coldest to hottest (see the lower image at left).

This is the horizon/homogeneity problem: Why is the temperature of the microwave background so uniform in all directions? This is really hard; the Universe is 13.7 billion years old, but was only about 380,000 years old when the background radiation was emitted. If different regions of the sky are now separated by up to 93 billion light years, how is it that they have temperatures that are so close to being equal to one another? After all, they’re so far away that they couldn’t have been in contact with one another, so they wouldn’t have a chance to thermalize, or achieve the same temperature. Look at the graphical illustration of this that I stole from UC Santa Cruz:

What the above image shows is that there are many different causally disconnected regions of space, that somehow have the same properties (e.g., energy density). If you ask how many, the answer is in the thousands. How do we solve this problem?

The only reasonable solution is to state that, somehow, these regions must have been causally connected at one time in the past, otherwise there’s no way for them to have the same properties. How is this possible? The Universe needed to expand, early on, by an incredible amount. What we can imagine is, just like we have “dark energy” now, that causes the Universe to expand without slowing down, we could have had it when the Universe was very young, only much stronger. This means that the Universe, in only a fraction of a second, could expand by an exponential amount, effectively increasing its size by a huge factor, like 10100! Even if it started out only as the size of a proton, this theory, known as inflation, says that the entire Universe could expand to be trillions and trillions of light-years in size, practically in an instant. So if you have this inflationary period near the beginning of your Universe,

you then end up with a Universe that should have the same physical properties everywhere, since everything was in contact with everything else prior to inflation. Does it sound crazy, or plausible, or annoying? There’s other evidence for it too, but this was one of the biggest arguments for it, and when Alan Guth discovered it, his paper was entitled “Inflationary Universe: A possible solution to the horizon and flatness problems,” a testament to how good Andy’s question really is!


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