Download Note Taking Guide #2: Characteristics of Stars Welcome back! As

Note Taking Guide #2: Characteristics of Stars
Welcome back! As you recall, before you left for your week at Walker Creek, we
completed a study (and activity) about stellar parallax. We learned that astronomers
estimate the distance to stars by a number of ways—parallax being a common means—
but no method is an exact measure of distance. Estimates are just that: approximations,
and, given the vast distances between objects in space, some approximations are more
reliable than others. In reading through your written descriptions of parallax in the study
and review questions I looked at, I noticed a big difference in the understanding of
parallax that was written on the page. Quite frankly, some of your descriptions were very
brief and underdeveloped. Others were satisfactory, and others were very complete.
Regardless of what you wrote, it is important that you understand the basics behind this
process of estimating distances to stars. If not, let Danny and me know and we’ll do what
we can to make it more understandable. Let’s move on.
Page 105
Stars differ from one another in many ways; however, astronomers commonly use the
properties of size, temperature, and brightness to classify stars.
Page 106 Star Size
Stars differ from one another in terms of their size, and astronomers group stars into
five groups based on this property:
Smallest stars 1.
2.
3.
4.
Largest stars 5.
Page 106 Star Temperature
Astronomers use a star’s color to determine its temperature. The color a hot object
gives off is a good indicator of its temperature. With stars, there is a difference between
the core temperature, which is usually the hottest part and the surface temperature, which
is the coolest part.
Red stars, like Betelgeuse, are the coolest stars.
Yellow stars, like our sun, are medium hot stars.
Blue/white stars, like Rigel, are the hottest stars.
Page 107 Brightness of Stars
Brightness, or luminosity, means “the amount of light an object gives off.” The more
light a star gives off, the more bright, or luminous, it is. The brightness of a star depends
on two factors:
1. Star size
2. Surface temperature
Large stars that are relatively cool (Betelgeuse) can give off as much light as smaller stars
that burn very hot (Rigel.) As a result, they both shine very brightly.
107 Apparent vs. Absolute Magnitude
How bright a star is from Erath depends on how far the star is from earth and how bright
the star actually is.
A star’s apparent magnitude is how much light a star appears to give off when viewed
from Earth. The closer a star is to Erath, the more light it appears to give off. Think of our
sun versus Alpha Centauri; the sun appears to give off much more light, even though the
two stars are actually very similar in size and temperature.
A star’s absolute magnitude is how much light a star would actually give off if it were a
standard distance from Erath.
Think of it this way: two similar flashlights, held side-by-side, and viewed by an observer
in the night from a long distance away would appear to have similar apparent
magnitudes. If they were a standard distance from the viewer, their absolute magnitudes
would be the same as well. Held two different distances apart from the viewer, however,
their apparent magnitudes would be different, but their absolute magnitudes would still
be the same.
Most stars’ luminosity is constant, but some stars’ brightness varies. If so, these stars are
called variable stars.
Page 108 The Hertzsprung-Russell Diagram
The realization that star color could be related to brightness came independently to two
astronomers at about the same time. The graph named after them shows the relationship
between the surface temperature and the brightness of a star.