What are the properties of stars

We can determine an amazing number of physical properties of stars, usually based only on a little bit of light. Over the next several lectures, ``we'' will talk about how the following properties are determined for stars.

Table of Contents Show

The Bigger Picture
What about star names?
Stellar Brightness
What are the three properties of stars?
What is a property of a star?
What are the most important properties of a star?
What are the 4 properties to classify stars?

BRIGHTNESS. This is not a fundamental property, but a combination of the luminosity and distance to a star (and in some cases also dependent on the amount of absorption in the direction of a star).
DISTANCE. From trigonometric and spectroscopic parallaxes. Determining distances to stars is how we figure out the scale of things in the Galaxy and is CRUCIAL to understanding stars because we can use the inverse square law for light dimming along with apparent brightness of stars to figure out how much energy is being produced and radiated away.
LUMINOSITY. This is the amount of energy generated in the star and released as electromagnetic radiation.
RADIUS
``Size''. From Stephan's Law.
CHEMICAL COMPOSITION. From the absorption line spectra. This one is tied up in a semi-complicated way with the next one.
TEMPERATURE. We have talked about Wien's Law and using colors to derive stellar temperatures, but, there are some complications. To REALLY get to surface temperatures of stars, we need to learn about and understand stellar Spectral Types.

The Bigger Picture

First - let's get oriented. We live out in the suburbs of a pretty good sized spiral galaxy that contains on the order of

stars.

The stars we see at night with the unaided eye are just the very tip of a big iceberg. Go out on a clear summer night somewhere where the city lights are not too bright and you can see a swath of light across the sky. This is the ``Milky Way'' - the light from thousands of unresolved stars in the plane of the Galaxy (galaxy with an uppercase ``G'' is the galaxy we live in).
The Galaxy (the one we live in is the Milky Way Galaxy) is one of the senior members of a small group. There are billions and billions of galaxies like the Milky Way out there.

What are the stellar Constellations?

Just people connecting dots. Many of the stars that are close together in the plane of the sky (the projection of the three dimensional galaxy into two dimensions) are quite far apart long the line of sight.

What about star names?

The brightest stars have various names - mostly from long ago. There are no ``official'' names for stars. The often used convention in the West is to use the Greek alphabet to identify the brightest stars belonging to some constellation.

Sirius
Canis Majoris
brightest star in the constellation Canis Majoris
Canis Majoris
2nd brightest star in the constellation Canis Majoris
There are also some pretty standard catalogues for the designations of bright stars and these are used world-wide.
You can start your own business selling star names and people have done so!

Stellar Brightness

I will use brightness to be the apparent brightness. This is different than luminosity with is an intrinsic property of a star - luminosity is the luminous energy produced in the core of the star. The brightness is a combination of the intrinsic brightness of a star and its distance (and sometimes also the amount of interstellar material in between). The brightest star in the sky is the Sun.
Astronomers use a logrithmic scale for the brightness of stars called the magnitude system. Every 5 magnitudes is a factor of 100 in intensity. It is confusing because it runs backwards. For example the Sun has a brightness of -23 magnitude. The next brightest star has a brightness of -1.4 magnitudes. Stars a factor of 100 fainter than this have a brightness of 3.6 magnitudes. In the city, this is about as faint a star as you can see. Out at a darker site, you can see stars as faint as 6th magnitude with the unaided eye. With a small telescope you can see a factor of 10,000 fainter to around 16th magnitude and with the big telescopes you can see another factor of 10,000 fainter to 26th magnitude stars.

Michael Bolte
Mon Jan 26 12:32:25 PST 1998

What are the three properties of stars?

There are three basic properties of a star that we measure: The Spectrum (or color) The Apparent Brightness (or flux) The Parallax (which gives us the distance)

What is a property of a star?

In general, a star is an object making its own energy and that is (or was) capable of sufficiently high central temperature and pressure that nuclear reactions can occur. Stars are so distant (the next nearest is 260,000 times further away than the sun) that we see them only as points of light.

What are the most important properties of a star?

Mass – The single most important property that determines other properties of the star. Luminosity – The total amount of energy (light) that a star emits into space. Temperature – surface temperature, closely related to the luminosity and color of the star.

What are the 4 properties to classify stars?

Astronomers use stellar classification in order to better understand the different types of stars in both our galaxy and our universe. Stars are classified based on a variety of characteristics including their temperature, mass, color, luminosity, and size.