Consider ‘Average - PowerPoint Presentation

Slide1

Consider ‘Average’ Stars

Slide2

Supremely Important

In order to make the most important astrophysical determinations, including

True (intrinsic) brightness

,

mass,

size,

e

tc

…

we

have to

know the distances.Slide3

It Sounds Easy![read the last three lines]

…b

ut

it’s actually not that simple! W

e will defer the actual ‘how’ while we consider a few relevant issues.Slide4

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?Slide5

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 6

th

magnitude.Slide6

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!Slide7

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. Slide8

What About Even Brighter Objects?As noted, the Greeks said the brightest stars were 1

st

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.Slide9

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:Slide10

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!Slide11

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 stereospeakers 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). Slide12

The Richter Scale is Similar

A magnitude 7 earthquake is bad; one of magnitude 8 is worse; but magnitude 9 is

much, much

worse!Slide13

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?Slide14

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.Slide15

Challenge: Find the Nearest Living Creature Next to YouSlide16

Not What You Might Think!Slide17

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!Slide18

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.Slide19

Consider Nine Conspicuous

Stars

Notice their

brightnesses

, sizes and distances

Name

True Brightness

(solar units)

Diameter

(solar units)

Distance

(light years)

Sirius

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

Rigel

60,000

40

810

Procyon

6

2.2

11

Achernar

650

7

120Slide20

Now Some Very

Nearby Stars

These are

not even visible to the unaided eye!

[Remember that we

can

only see

down to ~6

th

magnitude in a dark sky

.]

Name

Distance (light years)

Apparent Magnitude

Proxima Centauri

4.2

11

Barnard

’

s Star

4.9

10

Wolf 359

7.5

14

BD +36

o

2147

8.2

8

UV Ceti

8.8

12 / 13 (binary)Slide21

To Really Understand

Stars of All Kinds

…

…w

e 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. Slide22

What We FindThe

truly average star is

indeed

very

small and faint!

-- much less imposing than the sun.Slide23

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.]Slide24

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!