Lab 7 Magnitudes.pdf Astronomy Physics AY102

Lab Astronomy Magnitudes

1. Lab 7 - Magnitudes and the
H-R Diagram

2. Magnitude
Magnitude is a way of categorizing stars by their
relative brightness. For historical reasons, The lower
the number the brighter the star.

Negative numbers are brighter than positive. Sirius,
the brightest star in the night sky, has a magnitude of
-1.4, and the sun has a magnitude of -26.8.
1 2 3 4 5 6

3. Absolute vs Apparent Magnitude

Don’t forget, this is 10 raised to the power of what’s in the brackets, not multiplication.

4. The amount of energy a star emits depends on both its temperature and its surface area.

Temperature determines how much energy per unit of surface area the star emits. So, if there are two stars of the same
size, the hotter (bluer) one will have a lower magnitude.





Surface area determines how many of those units of surface area the star has. A larger star has more area to emit light
from than a smaller star. So, if there are two stars of the same temperature, the larger will have a lower magnitude.
A B
A B
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Remember: lower magnitude means more energy.

6. Spectral Types
Stars are divided into the spectral types which roughly group them by their temperatures.

The spectral types are OBAFGKM, which go from hottest/bluest to coolest/reddest. Like magnitude, the reason for this
strange ordering is a historical one. (Don’t worry, you won’t have to remember it!)

Each type is divided into subtypes, denoted by a number after the letter, for example, F0, F1, F2, etc. The higher the
number, the cooler the star is for its type.

You’ll often see a Roman numeral included to break down the classification even further. We can ignore them for this
lab.
O
>30,000 K
B A F G K M
10,000 – 30,000 K 7,500 – 10,000 K 6,000 – 7,500 K 5,200 – 6,000 K 3,700 – 5,200 K < 3,700 K

7. Color Index
As we learned in lab 3, for continuous spectra, the more
cool the more red and the more hot the more blue. But
how do we measure color?

When we observe things in space, we don’t look at the
whole spectrum. We look through different filters that
only collect light in a range of colors.

If we measure something in two different filters, we can
compare their magnitudes to find how much more red or
blue the object is. We call this number a ‘color index.’

For example, if we get the magnitude in blue light (the B
filter) and subtract the magnitude of green light (the V
filter), we get the B - V color index.

The smaller/more negative the number, the more blue
the object is.

8. The Hertzsprung-Russell (H-R) Diagram
On the x-axis is temperature, spectral type, or color
index (B-V, where smaller or more negative numbers
mean bluer). These all relate to the energy per area of
the star.

On the y-axis is luminosity, brightness, or magnitude.
These all relate to the total energy emitted by the star.

Stars lying along the “main sequence” are what we
consider a typical star. They are in the long-lasting
hydrogen burning phase. The hotter the star, the
shorter this phase.

As stars die, they expand and cool down, leaving the
main sequence and becoming giants or supergiants.
Their brightness does not greatly decrease, and can
even increase, because their surface area increases.

At the end of their lives, all that remains of the stars
are their hot but very small cores, called white dwarfs.