Robert P. Kirshner is Harvard College Professor of Astronomy and Clowes Professor of Science at Harvard University. He graduated from Harvard College in 1970 and received a Ph.D. in Astronomy[…]
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What’s floating around out there in the cosmological zoo? The Harvard astronomer describes the major objects visible via telescope and the naked eye.
Question: Can you provide a brief tour of the major objects visible in the universe?
Sure. If you look out at the nighttime sky, what you see right away are a few bright things that are planets in our own solar system or possibly the Moon, which is a terrible thing. We hate to have the Moon because all that reflected light makes the sky bright; makes it hard to do astronomy for distant objects. So, the real black belt astronomers don’t like the Moon; don’t like the planets very much. But, if you go out at night, you’ll see stars. And the stars that you see emitted their light, tens or hundreds, or even thousands of years ago. The speed of light, which everybody thinks is so fast, is really extremely slow. And it’s what lets astronomers see back into the past. So, for example, the speed of light is a foot, that’s a unit of distance, used here in the United States and I believe also in Myanmar. It’s a foot in a nanosecond, or a billionth of a second. So, you never see things the way they are, you always see the past. You always see light that bounced off somebody 10 nanoseconds ago, or in the back of a big room a hundred nanoseconds ago. When you go outside, you’re seeing light that was emitted – the sunlight that was emitted eight minutes ago, light in the solar system maybe up to an hour ago. And when you look at the stars, even the nearby stars, even the bright stars, the light has been traveling to your for tens of years, or hundreds of years, or even thousands of years. So, without a telescope you can see in the past a few thousand years. And what happened in the 1920’s was that people began to realize that the system of stars that we are in, which is the Milky Way; the Milky Way Galaxy we call it today, which we see as a band of light in the summer sky because we’re looking at this system, which is a big flattened system; kind of like a pizza edge on, except we’re a pepperoni. We’re on the pizza. And so our view of it is really quite awkward. We don’t have a good perspective of the Milky Way Galaxy. But we know now that it’s roughly speaking 100,000 light years in dimension across our Milky Way. So that means it takes light 100,000 years to travel across that span of distance. And that’s really just the beginning.
What people discovered in the 1920’s was that our galaxy is just one of billions of galaxies out there. The distances between the galaxies are a few times their own diameter. So, if the galaxy is this big, then the distance to the next galaxy is kind of ten times the size of the galaxy. So, if this is 100,000 light years, the distance to the next galaxy is a few million light years. And that’s fairly accurate. The galaxy that you can see – there’s one galaxy you can see without a telescope, if you know where to look, with binoculars. And easy object in a small telescope, and that’s the Andromeda Galaxy, M31. And in the autumn sky you can pick it out. It’s kind of a fuzzy patch. What we know is that is as big a system as the one we live in. It’s as big as the Milky way; it looks like a little tiny fuzzy patch because it’s so far away.
And that’s really just the beginning. That’s our local neighborhood a few million light years away. It turns out that with modern telescopes and the best instruments and the better detectors we have today, it’s not that hard to see things that are a few billion light years away, or to measure the light from them anyway. So, that’s a thousand times farther away, it means the objects appear a million times dimmer. But what has changed over time is that we have big telescopes that collect a lot of light, and we have detectors that are nearly perfect at measuring the light and turning it into an electronic signal. So, very similar to the detectors that are in digital cameras and so on; they are made of silicon they work pretty much the same way, but we take long time exposures and we add up the data very carefully.
Anyway, we’re able to make this – the technology has enabled us to make this leap so that we can study the distant objects. And the reason why we want to do that is that the telescope is really a king of no nonsense time machine. It let’s you see the way things were in the past. Of course, it doesn’t let you see into the future. It only lets you see the past, but we can do that to distances of a billion light years, and even with some effort, to many billions of light years. And that’s important because the time since the beginning of cosmic expansion, since the beginning of the universe as we know it, the time of the Big Bang, we think is about 14 billion years ago. So the biggest distance that light could travel in that time is about 14 billion light years. And we can see things most of the way back. That means we’re not just guessing that the universe has changed over time. The universe has expanded over time; has it gotten elaborated over time due to the action of gravity pulling stuff together and stars making more complicated elements and all that stuff that’s happened over the past 14 billion years is not just a story, it’s a real history that we can observe.
Recorded on February 17, 2010
Interviewed by Austin rnAllen
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