Module :

Activity 2:

From After the ZAMS

Module 8: Life on the Main Sequence

Swinburne Online Education Exploring Stars and the Milky Way

© Swinburne University of Technology

Summary

In this Activity you will learn about • the life of stars after they reach the ZAMS,• how the time they spend there depends on their mass,• whether stars gain or lose mass and why, and • open clusters and how they have taught us so much about the life stories of stars.

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Main Sequence

Temperature

ZAMS(Zero Age Main Sequence)

Our own Sun

Our Sun will spend about 80% of its entire life on the main sequence. What will happen then?

You’ll find out in the next Module!

While on the main sequence, though, the Sun will very slowly and quietly keep on turning hydrogen into helium.

10 billion years …that’s a nice long

middle age ...

10 billion years …that’s a nice long

middle age ...

Middle Age of a 1-solar-mass star

Helium is of course more compact than the four protons from which it was made.

So during this phase the core shrinks, which increases its temperature.

The radiated heat swells the outer layers: the star grows slowly larger and its surface becomes cooler.

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Temperature

Our Sun’s part ofthe main sequenceOur Sun’s part of

the main sequence

Sun movesa little onthe H-R

Sun movesa little onthe H-R

ZAMS

Watch out!

Many people get the wrong idea about stars and H-R diagrams, because of the term main sequence.

They think that stars move up the main sequence, but that isn’t so.

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Temperature

The sequence is actually a sequence of mass, e.g. more massive stars appear on the upperleft side and low mass stars on the lower right.

It’s got nothing to do with the history of any single star.

I reckon starsmove like this... I reckon stars

move like this... WRONG!! WRONG!!

What really happensYou’ll shortly see a rough map of how a star might move on the H-R diagram.

Its actual development will depend on its mass, its surroundings and its history.

During the animation, keep an eye on the chart below as well.

The important thing for you to remember is that stars don’t move up and down the main sequence. If anything, they move across it!

protostarprotostar main sequence starmain sequence star old starold star

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Temperature

Gaining mass?

gravity

gravity

Do stars gain or lose mass while they are on the main sequence?

You might think that because they are so incredibly massive that they would trap any gas or dust in the vicinity.

Come and havesome turkish delight ...

Come and havesome turkish delight ...

Er ... Er ...

Losing mass

A nice idea, but it forgets the stellar winds.Stars are constantly losing a bit of mass because of the matter-to-energy conversion in their cores.However the stellar winds are a much more serious source of mass loss.

These winds commence as soon as a molecular cloud starts to heat up.

But the effect really becomes important when fusion starts in the core.

A big problem

Stellar wind

Stellar wind

AAAARGHHHHH! AAAARGHHHHH!

But far greater than this is the loss of material from the star’s outer layers: it is just swept into space by the pressure of radiation from the core.

A large star can lose up to 60% of its mass in stellar wind.

All of the really light elements in its outer atmosphere can be blown away.

You mean I’m

leaking?

This means that anything near a star will be blown away, or eroded. If it moves anywhere, it will be most likely away from the star, not towards it.

Low mass stars

The less mass a star has, the longer it takes to evolve onto the main sequence and the longer it will spend there.

protostarprotostar main sequence starmain sequence star old starold star

sigh sigh

The Universe simply isn’t old enough for many very low mass stars to have made it onto the main sequence, let alone off it!

This slowness is however of great benefit to us on Earth, with a Sun which has evolved not terribly fast.

The Big OnesVery massive stars evolve very quickly, in stellar terms, and so we can see a lot of these at different ages and stages.

Remember too about look-back time: the further away a star is, the longer its light takes to reach us.

So our cameras may be receiving light (and therefore images) which left the star thousands of years ago. A 40 solar mass O5 star in the nearby galaxy Messier 31 will have evolved along the Main Sequence in 1/2 the time it takes light to travel to us!

For these reasons, we have a much better idea of the entire life story of the larger stars than we do of the smaller ones.

This is Betelgeuse, as seen recently by the HST.

As Betelgeuse is a red supergiant, it will probably explode fairly soon.

But as it is 520 light years away, we won’t know for 520 years!

Its time on the main sequence is very short

Its time on the main sequence is very short

The star turns hydrogen into helium more

through the CNO cycle than the p-p cycle

The star turns hydrogen into helium more

through the CNO cycle than the p-p cycle

There is likely to be carbon fusion and nucleosythesis of heavier elements in the core

There is likely to be carbon fusion and nucleosythesis of heavier elements in the core

The stellar wind can be so very fierce that it strips the outer layers of their lighter elements.

The stellar wind can be so very fierce that it strips the outer layers of their lighter elements.

There are also profound differences in the star’s old age (but they’ll be covered in a later Module).

There are also profound differences in the star’s old age (but they’ll be covered in a later Module).

A massive star is characterised during its time on the main sequence by a few clear markers.

Live fast,die young ...

Live fast,die young ...

Features of a Big One

A measure of age

While you can guess the age of a human by looking at their skin, hair, body shape and so on, with a star one of the best leads that you have is the proportion of hydrogen to helium and other elements.

I’m a B-class,but I haven’t gotmuch helium in

my core yet

I’m a B-class,but I haven’t gotmuch helium in

my core yet

You’re just a baby!I’ve been on the main

sequence so longmy core’s just

full of it ...

You’re just a baby!I’ve been on the main

sequence so longmy core’s just

full of it ...

A bit about Open ClustersYou’ll be studying globular clusters in a later Module, but we are now going to have a look at open clusters as they can tell us so much about life on the main sequence.

A cluster is open if its stars are pretty thinly scattered: usually 100 to 1000 stars in a space of a few tens of light years.

This is the Jewel Box cluster.

N330

Here is N330, a young cluster in the Lesser Magellanic Clouds.

Lovely, isn’t it?

Let’s look at it a bit more closely.

Colour

Blue = lotsof UV

(very hot)

Blue = lotsof UV

(very hot)

Red = H(Balmer series)

Red = H(Balmer series)

Pink = blue starswith lots of H

Pink = blue starswith lots of H

Yellow-white = A-typesupergiants

Yellow-white = A-typesupergiants

Orange = redsupergiants

Orange = redsupergiants

To tell the truth, the colours in this photo aren’t realistic.

They have been enhanced to indicate each star’s strongest radiation.

All in the familyHere’s a family like many human families: you can see the resemblances.

It is probably less likely thatthe boy will wear lipstick

and the girl will grow a moustache,but you never know ....

Looking at the parents allows you to predict what might happen as the children grow up.

It’s pretty likely that as they get older both kids will get darker hair.

Stellar familiesThe stars in open clusters are usually much like our own Sun: younger stars that are relatively rich in heavier elements being recycled from older, exploded stars.

The larger the star, the faster it will evolve. So a photo of an open cluster is like a family photograph. It will let us predict to some extent where a star is headed by looking at its faster-developing relatives.

Astronomers have learned a great deal about stars in general by studying such clusters, but we have to make a few assumptions.

1. Ageist assumptionsWe have to assume that all the stars in the cluster are of about the same age.

If we didn’t do that, we’d be unable to say as definitely as we do that larger stars evolve faster.

Having seen clusters forming in molecular clouds, though, supports the idea that stars in a cluster were “born” at about the same time.

This is the Lagoon Nebula, an active region in which clusters of

stars and protostars abound.

2. Compositionist assumptions

We have to assume that all the stars in the cluster are made of much the same kind of stuff: that they formed from the same molecular cloud, and it was well-mixed.

other2%He

26%

H72%

If we didn’t make this assumption, we’d be unable to predict what will happen to one star in the cluster by looking at others in the same cluster that are further along in their development. Composition

of our SunCompositionof our Sun

3. Groupist assumptions

Third, we have to assume that all the stars in the cluster are staying with the group and moving roughly in the same direction. The stars within the group may move around a bit, but the group as a whole heads in a common direction.

huphup

twotwo

threethree

fourfour

huphuptwotwo

threethree

fourfour

If we record the proper motion of the stars over a period of time, we can work out both the direction that the cluster is heading, and the motion of the stars within the cluster.

Within the group

Click to see that animation again and watch just one of the stars: you’ll notice that the stars within this particular group are circling each other.

huphup

twotwo

threethree

fourfour

huphuptwotwo

threethree

fourfour

What clusters tell usIf you make those three assumptions, you can really get down to business.

It is like having an entire family photo album so that you can study and predict the changes in a standard member over time.

A sort of time-lineIn this family, the hair apparently gets darker as middle-age approaches, then goes gray, then white.

At about the same time, the top lip disappears, sags appear under the eyes and the cheeks start to drop.

All through life the nose, ears and chin get bigger (but that’s true of all humans).

6 months6 months 7 years7 years28 years28 years

51 years51 years 74 years74 years

Stretching the truthThe trouble with these nifty analogies is that sooner or later they fall apart. In a human family, people are born at different times...

but (usually) age at about the same rate.

Life expectancyabout 80 years

Life expectancyabout 80 years

Life expectancyabout 80 years

Life expectancyabout 80 years

Life expectancyabout 80 years

Life expectancyabout 80 years

Life expectancyabout 80 years

Life expectancyabout 80 years

Life expectancyabout 80 years

Life expectancyabout 80 years

Born 1999Born 1999Born 1992Born 1992Born 1948Born 1948Born 1925Born 1925 Born 1971Born 1971

A cluster, on the other hand … In a stellar family, a cluster, the stars do the reverse.

They are born at the same time ...

… but they age at very different rates:the bigger, the faster.

2000,0002000,00015,00015,000

3,0003,000500500

33Born 1642Born 1642

Born 1642Born 1642Born 1642Born 1642

Born 1642Born 1642Born 1642Born 1642

Time on main sequence (millions of years)

Time on main sequence (millions of years)

(We chose 1642 for this examplebecause that’s when

Isaac Newton was born)

(We chose 1642 for this examplebecause that’s when

Isaac Newton was born)

Star family time-linesAstronomers treat photos of clusters as if they were family photos, and use them to look for changes in particular types of stars with time. However, human family members have their own peculiarities (like wearing lipstick or moustaches), and so do the stars in a cluster.

They are of different masses (and therefore at different stages), for a start.

They are subject to different stellar winds from neighbouring stars, different gravitational pulls, and different accidents (such as running into a nebula).

But we can still see a lot of patterns.

The PleiadesThis is the Pleiades cluster: a well-known, bright and beautiful group of stars in the constellation Taurus.

If you plot an H-R diagram for the Pleiades cluster,

Red line= ZAMSRed line= ZAMS

Spotty bit= PleiadesSpotty bit= Pleiades

temperaturetemperature

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you can see that different stars are at different stages of their evolution … because they are of different mass.

The Pleiades H-RAt the bottom, there are the small, newly-formed stars and protostars: cool and not very luminous, and very slow to reach the ZAMS.

protostarsprotostars

great hugebloated stars

that havealready leftthe ZAMS

great hugebloated stars

that havealready leftthe ZAMS

Once a star joins the ZAMS, it usually becomes a bit brighter and cooler, so the middle-sized stars drift up and right of the ZAMS.

The very large stars, with the highest temperature and luminosity, are so mature that they are leaving, or have already left, the ZAMS.

(The next Module looks at those.)

PLEASE NOTE: all of the stars in this diagram are the same age!

small starssmall stars

middle-sized stars

middle-sized stars

Summary

This Activity has shown you how stars of different masses continue to evolve very slowly after joining the Zero Age Main Sequence.

The evolution of the star will be controlled mostly by its mass.

Open clusters give us a sort of “family album” that lets us see stars of the same age and type (but different mass) at different stages of their evolution.

Image Credits

MSSSO: Michael Bessell © (used with permission)

The Jewel Box cluster (NGC 4755), containing Kappa Crucis

N330

Lagoon Nebula

RCW 38:

http://antwrp.gsfc.nasa.gov/apod/image/9812/RCW38_vlt_big.jpg

AAO: Pleiades cluster © David Malin (used with permission)

http://www.aao.gov.au/local/www/dfm/image/uks018.jpg

Now return to the Module home page, and read more about stars on the main sequence in the

Textbook Readings.

Hit the Esc key (escape) to return to the Module 8 Home Page