Last week, I teased you with the idea that it’s actually easy to estimate distances to galaxies.
I do mean estimate–and distance indicators are still important.
The Hubble Law is named for Edwin Hubble, the astronomer who was first able to settle the debate over what galaxies were–using the new Hale Telescope, the largest in the world at the time. But the Hubble Law is undoubtedly what he’s most famous for.
In order to understand the Hubble Law, though, we first need a little review of the Doppler effect…
Way back when we spent a number of posts surveying the stars, we covered binary systems. These are star systems that contain multiple stars. Imagine if our sun had a companion, and two stars rose and set in our sky over the cycle of day and night.
It might surprise you that the majority of stars in the universe are actually in binary systems. Our solar system seems to be an outlier in that regard. Most stars have a companion or two or six…
…and so do some neutron stars.
Remember that neutron stars are the collapsed remnants of massive stars that have gone supernova. If most stars are part of binary systems, then naturally, some of these stars will evolve into neutron stars and still be part of their birth system.
Meet the planetary nebula, one of the universe’s most gorgeous phenomena.
If you’ve ever looked through a telescope, you may have seen one of these before. Through a small telescope, one might look like a little planet—hence the name. But make no mistake, these nebulae have nothing to do with planets, and everything to do with stars.
Then, as the star runs out of helium fuel in its core, it expands into a giant a second time. This is the last time a medium-mass star will expand. It’s also the end of the line for the fuel in its core, since it can’t get hot enough to fuse carbon.
At this point, the star is so big that gravity at the surface is too weak to hold onto its atmosphere, especially in the face of the superwind of radiation pressure from the still-collapsing core.
The result is a planetary nebula…but what exactly is a planetary nebula? What is it made of? Why does it look the way it does?
Consider a solar system far different from our own. A solar system governed by two suns, and consisting of planets we can only dream of.
Would it surprise you to hear that, based on recent discoveries, that might actually be the norm?
The surroundings we grow up in determine our outlook on the world, and this is never more true than with our solar system. Our eight planets (though some would vehemently insist upon nine) and their parent star are all we know.
But what if I told you that most of the stars you see when you look up at the night sky have companions? And often, these companions are impossible to detect by visual means.
Albireo is the distinctive double star in the head of the constellation Cygnus. You can find it yourself if you look for the Summer Triangle amid the dusty trail of the Milky Way across the night sky.
The brighter, orange star of Albireo is a K3-class bright giant. That means it’s just a few thousand Kelvins (Celsius degrees plus 273) cooler than the sun. But it’s also larger—70 times the sun’s radius—and that makes it brighter than you would expect.
The blue star, on the other hand, is a B8-class dwarf. It has only about 3.5 times the sun’s radius, although it’s hotter by about 7422 Kelvins.
Neither star in Albireo is particularly unusual. There are doubtless millions, even billions, of other stars similar to each one. But Albireo certainly offers us the most striking contrast. Bright blue and red stars don’t often appear so close together.
But what exactly gives these stars their distinctive colors?
Well, at the time stamp of about 2000 AD (CE), I think you will. It’s one of the most famous constellations in the night sky.
Well, technically, it’s not a constellation at all.
It’s an asterism—a commonly recognized grouping of stars that isn’t actually official as a constellation. There are tons of asterisms that you no doubt recognize…the Summer Triangle, the Great Square of Pegasus, the Big Dipper.
That’s right. That mess of stars up there that keeps changing for some reason…that’s the oft-recognized Big Dipper, part of the constellation Ursa Major.
That makes sense, really. We can’t see below the surface of the Earth, either—we have to get creative if we want to find out what goes on below the crust.
In the sun’s case, we can’t see below its photosphere because the gases within are so dense, light can’t escape. And we depend on light to see anything.
So…if we can’t see inside the sun, how do we study it?
You might, if you’ve ever seen the sun through a telescope before. What you’re seeing is the photosphere, the layer of the sun whose light reaches Earth. This is the only layer you’ll ever see, without the aid of a solar eclipse.
Wait a second…what do I mean, layers? I mean, I know what a layer is, but what kind of layers does the sun have?
When I was little, I remember hearing the ice cream truck all the time. Just the sound of the opening notes of “Pop Goes the Weasel” were enough to propel me to the door, where I’d beg my parents to let me go out.
Of course, I didn’t always make it out front in time. But one day, my dad found a way to solve that problem—by actually getting in the car and chasing the ice cream truck.
I remember us driving around the neighborhood, following that white truck around. A few times, it slowed and stopped, but when we stopped too, it kept going again. It took a while for the driver to realize we were following him!
Eventually, we caught it, and had a good laugh over it. But the moral of the story is…have you ever noticed that you can tell if something is moving toward you or away from you, just by if it’s getting louder or quieter?
Cosmos at Your Doorstep 