About this video
This short correction fixes an earlier claim about quasars. Quasars do not emit mainly in X-rays. Their accretion disks radiate across the whole electromagnetic spectrum, from gamma rays down to radio waves. Because quasars are so distant, their light is redshifted by the expanding universe, which stretches it toward lower frequencies, and that is why the earliest observations were in the radio band.
You'll be able to
- state that quasars emit across the whole electromagnetic spectrum, not just X-rays
- explain how cosmological redshift shifts a quasar's light toward lower frequencies
- explain why quasars were first detected as radio sources
Key words
- Electromagnetic spectrum
- The full range of electromagnetic radiation, from gamma rays and X-rays through visible light to infrared and radio waves.
- Redshift
- The stretching of light to longer wavelengths and lower frequencies, caused here by the universe expanding.
- Frequency
- How many wave cycles pass a point per second; higher frequencies mean more energetic radiation.
- Accretion disk
- A disk of matter spiraling into a black hole, heated to emit radiation.
- Correction
- A revision to an earlier claim, here about the energy band in which quasars mainly shine.
Transcript
I want to make a quick correction to the video on quasars. In that video I said, and I mistakenly said that the creation disk that's really releasing the energy of the quasar, that it's releasing energy predominately in the x-ray part of the electromagnetic spectrum.
And that was incorrect. Most quasars are actually emitting electromagnetic radiation across the spectrum, all the way from x-rays, as high frequency as x-rays, all the way down to infrared. And some quasars even release super high frequency gamma rays, and they'll release low frequency electromagnetic waves all the way down to radio waves.
So I just wanted to make that correction. It's not predominately in the x-ray part of the spectrum. It's across the spectrum right over here. It's this entire range of the spectrum, and sometimes even a wider range. Now, the other thing I want to clarify is this is the range of the spectrum that's being emitted.
But we have to remember that most, or actually all of these quasars are quite far away. The closest is 780 million light years away. Many of them are many, many billions of light years away. And so they're moving away from us at a very fast speed, or they're getting redshifted because the universe is expanding so fast relative to us at that point, or that coordinate is moving so fast away from our coordinate.
And so even though this is the spectrum that's being emitted, it's all going to be redshifted. It's all going to be redshifted down, and so we are going to observe things at a much lower frequency, maybe around the radio part of the frequency. So everything will be redshifted down.
And that's why these were originally called quasi stellar radio sources. Anyway, hopefully that clears things up a little bit.
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