1-5 Properties of Stars

Distances Beyond the Solar System

Within the Solar System, the astronomical unit (AU) is a handy unit to measure distance. However, beyond our Solar System, measuring distances in AU would be like trying to measure the distance to the moon using a 30-cm ruler.

We have a unit called a light year (LY) which is much more appropriate for these larger distances. (Having "year" in the name might suggest it's a unit of time, but it's distance.) It goes like this:

• The speed of light, c, is equal to about 300 000 km per second.

• This speed never changes, ever -- so it is a reliable way to measure things.

• In physics, (speed) × (time) = (distance travelled).

• If we take (speed of light) × (1 year), the distance travelled will be 1 LY.

So, if it takes 2 years for light to travel from one place to another, that means the distance between those to places is 2 LY. Here are some typical distances in terms of light-years:

Our local collection of stars, the Milky Way galaxy, is something we're embedded in. We aren't near the centre, but we aren't at the very edge. Based on our observations, the following is what we think the Milky Way looks like from the outside -- we've never been outside it -- and here's how far we are from the centre:

Luminosity

This light bulb emits light:

The power rating on this bulb is 7 W, which means it takes in 7 J of energy every second. Its efficiency is roughly 45%, which means it emits about 3 J of light energy per second, or 3 W of power output.

The Sun also emits light:

As you might imagine, it emits far more energy than a light bulb you'd find in a house. We have measured the luminosity (L) of the Sun to be about 3.8 × 10²⁶ W.

That's a lot.

Astronomers find it handy to refer to the luminosity of other stars in terms of the luminosity of the Sun. Because this is something that is used often, it's given a symbol: L_⊙.

The Sun gives off a lot of light, but as far as stars go it's pretty average. The next-closest normal-sized star to us, Alpha Centauri A, has a luminosity of about 1.52L_⊙. Its twin, Proxima Centauri, has a luminosity of 0.00005L_⊙ -- much less than the Sun.

On the other end, there is a star relatively close to us in our galaxy, Eta Carinae A, which is in the middle of blowing itself up. It is located about 7500 LY away from us, and its luminosity is about 5 million times that of the Sun.

On the left is a picture of the star, which started exploding in the 1830s. On the right is an animation of an ejection of some material from Eta Carinae. Who knows what this weird star has in store in the years to come!

Star Colours and Temperatures

Stars can appear as different colours in our sky. This is a slightly-enhanced field of stars in our night sky, as seen by the Hubble Space Telescope:

Astronomers have known for centuries that stars are different colours, and many stars had been classified into different spectral classes, based on what colour they appear.

However, astronomers didn't know why stars appeared these different colours until the 20th Century. But we now know that it's because these stars are different temperatures.

While scientists usually like the Celsius scale for temperatures, it's inconvenient because sometimes there are negative numbers. Instead, the Kelvin scale is often used. Without getting into too much detail, take the Celsius temperature, add 273, and that's the Kelvin temperature.

Here's a rough correspondence between colour and temperature:

Spectra

A spectrum (plural: spectra), when talking about light, refers to the spreading-out of "white" light into the colours of the rainbow that make it up.

Here, a triangular "prism" of light does the separating-out; this phenomenon was first noted by Isaac Newton in the late 1600s.

If you spread the light from the Sun out into a rainbow of colours and you look very carefully, it isn't as smooth a rainbow of colours as you might expect:

Like the idea of star colour and temperature, it took a while to unlock the meaning of the missing colours in the Sun's spectrum.

If you take a glass tube full of low-pressure hydrogen, put a few thousand volts across the electrodes at the ends of the tubes, and turn out the lights in the room, this gas discharge tube glows with purple light:

But, if you spread this light out into the colours that make it up, you only get a few colours of light instead of a rainbow:

Light from the Sun travels out in all directions, including towards Earth. But to travel outwards from the Sun's visible surface, it has to travel through the corona, the Sun's atmosphere:

The Sun's visible surface glows with all colours of the rainbow. But the light that makes it to Earth, because it has to pass through the corona -- which includes hydrogen -- has colours missing, as seen above.

The colours missing from the full rainbow, due to hydrogen, are the same as the colours emitted by the hydrogen in the gas discharge tube:

If we look at the entire rainbow, we see many colours missing, and each of those colours corresponds to a different element present in the corona and, therefore, in the Sun.

The study of spectra, spectroscopy, finds its way into a wide variety of sciences: physics, chemistry, biology and earth science, among others.

Practice

The Basics

1. Explain the connection between a star's colour and its surface temperature.

2. What is a "light year?"

3. How do the missing colours in the Sun's spectrum connect to the elements present in its atmosphere?

Extensions

1. The element helium was not discovered on Earth first.

a. Find out what it's named for.

b. Find how it was discovered.