Telescopes and estimating the distances to astronomical objects
Why do we use telescopes? 1. Light-collecting area: A telescope is a light bucket
Q: How much more light can a telescope with a diameter of 2m collect than a telescope with 1m diameter? A. The same amount B. Twice as much C. Three times as much D. Four times as much
Q: How much more light can a telescope with a diameter of 2m collect than a telescope with 1m diameter? A. The same amount B. Twice as much C. Three times as much D. Four times as much
Why do we use telescopes? 1. Light-collecting area: A telescope is a light bucket The light-collecting power goes like the diameter 2 2. Angular resolution: a large telescope has the ability to resolve objects even if they are very close together on the sky (i.e. it produces shaper images)
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Two kinds of telescopes: refractors Refracting telescopes are the simplest kind, and they were also the first: this is what Galileo built. These telescopes use a lens to focus the light.
Two kinds of telescopes: refractors The problem is that it s hard to build a really big lens, and then the telescope becomes really long! 1m refracting telescope (the largest in the world) in Wisconsin. It was build in 1895.
Two kinds of telescopes: reflectors Reflecting telescopes use mirrors to reflect light rather than lenses to refract (bend) light. This makes them much easier to build!
Two kinds of telescopes: reflectors The famous 5m Hale Telescope in California.
Two kinds of telescopes: reflectors In the old days, the astronomer had to sit in a cage at the prime focus all night to switch out the photographic plates.
Two kinds of telescopes: reflectors Modern reflecting telescopes have a secondary mirror that focuses the light through a hole in the primary mirror.
Two kinds of telescopes: reflectors A 10m mirror (one of the largest) in Keck Telescope, on top of a mountain in Hawaii
Two kinds of telescopes: reflectors
Where do we put telescopes? The observatory on Mauna Kea, at 14000 feet
Where do we put telescopes? The observatory on Mauna Kea, at 14000 feet
Where do we put telescopes Astronomical observatories are typically built on the tops of mountains Away from light pollution Above (some of) the weather Above (some of) the distorting effects of the atmosphere
The distorting effects of the atmosphere Turbulence in the atmosphere causes the light from objects to jump around very rapidly, and also makes them appear brighter and fainter. This is why stars twinkle. This makes astronomical images blurry. But if you can get above the atmosphere, the images will be sharper.
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The Hubble Space Telescope Edwin Hubble (1889-1953)
The Hubble Space Telescope
What about the other wavelength regimes?
What about the other wavelength regimes? The Earth s atmosphere is transparent at visible wavelengths (and at radio wavelengths). That s why we can see! And that s why radio works! Observing at other wavelengths requires going to space
The Chandra X-ray Observatory Subrahmanyan Chandrasekhar (1910-1995)
The Spitzer Space Telescope (infrared wavelengths) Lyman Spitzer (1914-1997)
What about the other wavelength regimes?
What about the other wavelength regimes? The atmosphere is transparent at visible and radio wavelengths
Radio telescopes Arecibo Observatory in Puerto Rico (300m diameter)
Radio telescopes Karl G. Jansky Very Large Array (JVLA), New Mexico
Distance measurements: parallax As the observer moves, the angular position of a nearby object will shift. We call (half of) this shift the parallax angle
Distance measurements A standard candle is an object that has known luminosity. If we know the luminosity, and we can measure it s apparent brightness, then we can figure out it s distance: apparent brightness = luminosity 4π distance 2 Two of the most important types of standard candles are: Cepheid variable stars Type Ia supernovae
Distance measurements Cepheid variable stars pulsate, getting brighter and dimmer with time. The period of this pulsation is related to the star s luminosity
Distance measurements Cepheid variable stars pulsate, getting brighter and dimmer with time. The period of this pulsation is related to the star s luminosity We can easily measure the period of cepheids in nearby galaxies, which tells us the luminosity. And since we can also easily measure the brightness, this gives us the distance. Leavitt s law after Henrietta Swan Leavitt (1868-1921)
Distance measurements Type 1a supernovae are a special class of supernovae (caused by an exploding white dwarf) that always have the same luminosity, so they are also excellent standard candles. And because they are so bright, we can see them from very far away
Distance measurements The distance ladder describes how the process of estimating the distances to ever-further objects: We can use parallax to figure the distance to nearby cepheid variable stars. Then we can use that distance and the apparent brightness to calibrate Leavitt s law. Then we can find cepheids in nearby galaxies and estimate to estimate their distance. If we see a Type 1a supernova in one of those galaxies, we can calculate it s luminosity. Then if we see Type 1a supernovae in very distant galaxies, we can estimate their distances.