Transcript Consider Average Stars

Consider ‘Average’ Stars
Supremely Important
In order to make the most important
astrophysical determinations, including
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True (intrinsic) brightness,
mass,
size,
etc…
we have to know the distances.
It Sounds Easy!
[read the last three lines]
…but it’s actually not that simple! We will defer the actual
‘how’ while we consider a few relevant issues.
1. Stellar Brightness
The apparent brightness of a star – that is, what we
actually see – is partly an accident of location: nearby
stars can look deceptively bright. (The obvious example
is the Sun!)
But the intrinsic (true) brightness of a star is a good
measure of how much energy is being generated, how
fast the fuel is being consumed, etc. So it’s something
we really need to know.
Question: How do astronomers describe the brightness of
stars?
First: Apparent Brightness
The ancient Greeks divided the stars into six broad
categories, called magnitudes.
This depended on how soon they appeared in the evening
as the sun was setting and the sky darkened. The
brightest ones (“first magnitude”) show up first, then
the “second magnitude” stars, and so on.
The faintest stars visible to the unaided eye are about 6th
magnitude.
Star Light, Star Bright…
You can watch this behaviour using the Starry Night simulation software.
Look to the western horizon, with the sun still up in the sky. Then move
your cursor to the time display at the top left, and advance the minutes
quickly by holding down the arrow key on your keyboard. This makes
the sun set rapidly – and you can watch the stars come out!
A Special Scale
Stars with numerically larger
magnitudes are fainter
[this perplexes some people!]
The scale measures ratios
of brightness: if one star
is 5 mag brighter than
another, it is 100x as bright.
Note that this illustration suggests that brighter stars are
perceptibly bigger, but this is not the case. Except for the
sun, essentially all the stars in the sky are completely
unresolved (points of light) even through large telescopes.
What About Even Brighter Objects?
As noted, the Greeks said the brightest stars were
1st magnitude. Some objects (Venus, Jupiter, the
Moon, …) are even brighter – so their magnitudes
are negative numbers.
(And why not? We are not upset when the temperature
outside is negative! The numbers still make sense.)
Example:
Vega is a “first magnitude” star. But Venus,
at its brightest, is 100x (5 magnitudes) brighter. So Venus
is magnitude -4.
Real Stars
We now do better than the Greeks, measuring star
brightnesses very precisely – so there are, for example,
stars of magnitude 2.50, or 3.75. They are not merely put
into ‘groups.’ Here are some real examples:
A Huge Range!
The fantastically bright Sun is at magnitude -26.
(The full moon is at -11.)
The faintest star seen through the world’s largest
telescope is magnitude 29. (It could, of course, actually
be a very bright star, but extremely far away!) This is 55
magnitudes fainter than the Sun appears to be.
Every 5 mag ‘step’ corresponds to 100x more light. So
the Sun gives us 100 x 100 x …. X 100 (11 steps)
= 10,000,000,000,000,000,000,000x
as much light as that faint star!
Note the Convenience!
Magnitudes are small numbers that allow us to depict a
huge range of brightness in very concise form.
Historically, they arose because of the fact that our human
perceptions respond equally to equal ratios of stimuli.
(Fechner’s Law).
Example: Double the power in your stereo
speakers from 10 to 20 Watts. The music
now sounds louder. But adding 10 more
Watts doesn’t have the same impact!
Instead, you have to double it again (from
20 to 40 W).
The Richter Scale is Similar
A magnitude 7 earthquake is bad; one of
magnitude 8 is worse; but magnitude 9 is
much, much worse!
2. Identifying ‘Average’ Stars
Let’s go out at night, look up at the sky, and draw
up a list of every star we can see.
Now work out their distances [more on this later]
to find out what these stars are like intrinsically.
Does this give you some idea about the‘average
star?’ Or are these stars atypical in some way?
Night-Time Sky – Lots of Stars
[and, in this picture, two very bright planets!]
Note also the Pleiades star cluster to the upper right, and cool (red!)
Betelgeuse near the top center.
Challenge: Find the Nearest Living
Creature Next to You
Not What You Might Think!
There is a Strong Bias
In the lecture hall, or out in the field, we notice the big
animals! Meanwhile, myriads of microscopic
bacteria and large numbers of tiny bugs swarm
unseen all around us.
They are the truly
average living
creatures!
Similarly the Stars
Could there be myriads of stars so faint that they
are not even visible to the unaided eye? They
could be all around us, in their thousands, but
simply languish unnoticed!
In fact there are such stars! They are the ‘bugs
and microbes’ of the astronomical world.
So the “obvious approach” of studying the
prominent stars is misleading.
Consider Nine Conspicuous Stars
Notice their brightnesses, sizes and distances
True Brightness
(solar units)
Diameter
(solar units)
Distance
(light years)
23
2
9
Canopus
1400
30
110
Arcturus
115
25
36
Alpha Centauri
1.5
1.1
4.5
Vega
58
3
27
Capella
90
13
46
60,000
40
810
Procyon
6
2.2
11
Achernar
650
7
120
Name
Sirius
Rigel
Now Some Very Nearby Stars
Name
Distance (light years)
Apparent Magnitude
Proxima Centauri
4.2
11
Barnard’s Star
4.9
10
Wolf 359
7.5
14
BD +36o2147
8.2
8
UV Ceti
8.8
12 / 13 (binary)
These are not even visible to the unaided eye!
[Remember that we can only see down to ~6th
magnitude in a dark sky.]
To Really Understand
Stars of All Kinds…
…we have to find all the stars in the solar
neighbourhood, even the little faint ones! This
requires telescopes, and many years of survey
work.
We have to determine their distances, to work out
how bright they are intrinsically.
What We Find
The truly average star is indeed very small
and faint! -- much less imposing than
the sun.
One Important Lesson
For every big thing in Nature, there are lots of little things.
[There are thousands of blue whales, but billions of people, trillions of
ants, and uncountable hosts of microbes.]
Similarly in
Astronomy
On the moon, there
are a few big craters,
but many more
little ones.
In the Solar System, there are a few really big
asteroids, but trillions of pebbles!