An Introduction to Astronomy

Part XI: The Birth and Death of Stars

Lambert E. Murray, Ph.D.

Professor of Physics

Interstellar Gas and Dust

In the late 1700’s Henry Herschell discovered “holes in the heavens” where there appeared to be fewer stars than normal.

In the late 1800’s Edward Barnard’s photographs of these regions lead some to believe that they were clouds of material blocking out the starlight.

Barnard 86 – a Dark Nebula

Barnard 86 is a good example of one of

Herschell’s “holes in the heavens”.

The ConstellationOrion

Region of Horsehead

Nebula

The Horsehead Nebula:

A Dark Nebula

Close-up of the Horsehead Nebula

Evidence of Dark Nebula

The Horsehead nebula is clearly a case where dark material is obscuring the brighter emission nebula behind it.

The close-up actually reveals dim stars that are behind the dark nebula.

This seems to be clear evidence for dark material in interstellar regions which can obscure the light from stars.

Indeed, the reddish emission nebula behind the Horsehead is direct evidence of interstellar gas.

CO Radar Mapping (2.6 mm)of Orion – Monoceros Region

The following image is a radar map of the Orion – Monoceros region of the sky taken at a wavelength of 2.6 mm.

The 2.6 mm wavelength radar image maps CO concentrations. The concentrations of hydrogen are typically four orders of magnitude greater than CO in interstellar space, but this gives a measure of the amount of gas in interstellar space regions.

You can tell that there are large concentrations of gas in the regions of the horsehead and Orion nebulae. These are believed to be rich star formation regions.

M

More Dark Nebula

Emission and Reflection Nebula The term nebula is used to denote a cloud of

interstellar gas and dust. An emission nebula is one that glows because of the

emission of specific spectral lines arising from the excitation of atoms within the interstellar gas cloud.

A reflection nebula is one that glows because of scattered light from a star – this scattered light is typically bluish in color (like the blue in our atmosphere).

As white light passed through a dust cloud (much like smoke) blue light is scattered and, if the cloud is not too thick, the spectrum of the light passing through the cloud becomes more reddish (interstellar reddening).

An Example of an Emission and Reflection Nebula

Reflection NebulaNGC 6589NGC 6590

Emission NebulaIC 1283-4

Extinction and Reddening Due to the interstellar nebula, light from distant

stars does not appear as bright as they would – the process is called interstellar extinction.

Similarly, the interstellar nebula cause the light from distance stars to appear more reddish – interstellar reddening.

Measurements of interstellar extinction and reddening in various directions indicates that most of the interstellar nebula are confined to the regions of the Milky Way – the faint band of hazy myriad stars which stretches across the night sky.

The Milky Way is a Spiral GalaxyWe are Near the Outer Edge

Looking Toward the Nucleus of Our Galaxy– Through Dark Nebula

The Formation of Protostars

Astronomers believe that stars are born from the gravitational collapse of large, cold regions of dark nebular material. This collapse may be triggered by the explosion of nearby stars creating compression waves in the nebula.

The picture on the next slide is a region of space where astronomers believe this process may be occurring.

The Knobs on the Gas Clouds May be Regions of Concentrated

The Orion Nebula

Protostar Region?

Pre-Main-Sequence Evolution

As large regions of gas collapse under the influence of gravity, they heat up.

Initially this heat is in the form of infrared radiation.

Eventually the gas heats up enough to radiate in the visible. As the gas cloud shrinks greater internal energy is released, causing the temperature to rise more.

A Stellar Nursery

H II region of the Swan in visible wavelengths

Same region in infrared wavelengths.

Notice the large number of “cool” stars, or protostars, on the right-hand-side. These are visible in the infrared because that

wavelength can penetrate the dust and gas.

These Two Images are Lined up

These Two Images are Lined up

Pre-Main-Sequence Evolution Tracks of Protostars

Notice that this model gives results similar to the mass-luminosity data plot.

Variations in Luminosity with Stellar Mass

The rising temperature coupled with the decreasing size causes protostars of mass greater than about 5 solar masses to maintain a relatively constant luminosity.

Protostars less massive decrease in size more rapidly than the increase in surface temperature, and the luminosity decreases.

A Star is Born Once the protostar heats up to the point where

thermonuclear fusion occurs, the radiation pressure will counteract the gravitational pressure and the star will become stable as a main-sequence star.

Stars arising from larger mass clouds become very luminous stars, while stars arising from less massive clouds become less luminous.

Gas clouds with a mass of less than about 0.08 solar masses can never heat up sufficiently for nuclear fusion to occur, and the failed star becomes a hydrogen-rich brown dwarf (something like Jupiter).

Young Stellar Disks and Jets From the discussion of our solar system, we

postulated the formation of a solar system from the gravitational collapse of a dust and gas cloud, and the development of a disk of material rotating rapidly around the young star.

Similar features have been recently observe with the Hubble Space Telescope.

In the slide that follows, the knobby jets of material appears to be emitted along the polar axes of the star in what is called bi-polar outflow. These small nebula are known as Herbig-Haro objects.

Such young, gas-ejecting stars are known as T Tauri stars, the first example having been discovered in the constellation Taurus.

Young Star Jets and Disks

Stellar Disks in the Orion Nebula

The following images were taken by the HST of the Orion Nebula.

In these pictures you will see evidence of stellar formation and of the presence of disk-like structures surrounding these new stars.

The four massive stars that dominate this region are emitting radiation and gasses which are interacting with the smaller young stars being formed. These stellar winds may prevent the formation of possible solar systems.

Main Sequence Stars Once a star reaches the main sequence, it will

remain on the main sequence for about 90% of its lifetime. For our Sun (a moderately sized star), this means approximately 10 billion years.

Larger, more luminous stars burn up their fuel much more rapidly, and remain on the main sequence for a shorter period of time.

As we will see later, when a star leaves the main sequence, it expands and moves toward the region of giant and supergiant stars.

Star Clusters Provide Evidence for our Model

High-mass stars evolve more rapidly than low-mass stars.

An association of hot, massive stars (an OB association) will emit vast quantities of uv radiation into the nebula from which it was born.

This high-energy uv radiation actually ionizes the hydrogen gas of the nebula. These free protons then combine with free electrons and emit the characteristic red color associated with these emission nebula (so-called H II regions).

We want to examine the HR diagram for the young stars associated with an H II region (NGC 2264).

NGC 2264in

Monoceros

HR Diagram of the Young Star Cluster NGC 2264

Note that the more massive, luminous stars in the cluster (at the upper left end) have already reached the main sequence, while the less massive protostars (at the lower end) have not yet joined the main sequence.

The Pleiades Star Cluster

In contrast to the young star cluster NGC 2264, an HR diagram of the Pleiades star cluster shows that these stars have already reached the main sequence, and the older stars are actually beginning to leave.

The Pleiades star cluster is a cluster of very bright, blue (hot) stars.

Both star clusters we have examined are known as open clusters or galactic clusters, possessing barely enough mass to hold themselves together in a cluster.

The Pleiades and Their HR Diagram

Other Stellar Nurseries The radiation pressure from very bright stars may

create compression waves in a surrounding nebula, as in the Rosette Nebula.

Likewise, when a star dies, it may generate a massive explosion which can send large compression waves out into the interstellar medium, compressing the gasses that are there.

This compression of interstellar gasses may be the breeding places for new stars.

The following images illustrate some of these massive compressional waves generated by radiation pressure and by exploding stars.

The Rosette Nebula: Compression by Radiation Pressure

An Exploding

Star

Another Exploding Star

Yet Another

Exploding Star

The Sun Expands in Old Age Once a star like our Sun becomes a main sequence

star, it remains stable for about 10 billion years. At the end of that time, the hydrogen fuel in the

center of the Sun will become depleted; there is too much helium to efficiently continue the thermonuclear fusion process at the core.

When that happens, the radiation pressure from the center of the Sun will be reduced and the core will collapse toward the center due to gravity.

The region just outsider the core will heat up and begin to “burn” hydrogen and this will cause the Sun to expand. This process continues as the Sun expands out to the size of the Earth’s orbit, creating a red giant.

The Sun as a Red Giant

Helium Core Burning Once a star becomes a red giant, it will remain a red

giant as its outer regions continue to burn the available hydrogen.

During this time, helium ash continues to accumulate in the center of the star as gravity pulls this heavier material to the center, heating up the core.

Eventually, the star’s core will re-ignite when the temperature of the core gets hot enough for helium to “burn”.

The ash of helium burning is both oxygen and carbon. For low-mass stars (less than 3 solar masses) the onset

of helium burning produces an explosive “helium flash”.

After the helium flash the star settles down to burning helium and becomes a smaller, hotter star.

Post-Main-Sequence EvolutionCore helium burning begins where the evolutionary tracks make a sharp downward turn in the red giant region of the diagram.

HR Diagrams for Globular Clusters

Many globular clusters are associated with our galaxy (and with others).

These star clusters can be easily analyzed using an HR diagram, because all the stars are essentially the same distance from us. We need plot only their apparent magnitude vs. their temperature.

An HR diagram of M13 is shown on the next slide.

This diagram indicates a group of stars in which the hottest stars have already moved off the main sequence. This star cluster, therefore, must be relatively old.

HR Diagram for M13

The Horizontal Branch

The horizontal branch stars in this last HR diagram are believed to be stars that have already experienced the helium flash.

Note the “gap” in the horizontal branch. This is the region occupied by the variable stars.

Composite HR Diagrams for Several Star Clusters

The turn-off point for each cluster gives us an estimate of the age of each star cluster. Notice the “gap” in the central part of this diagram.

The Instability Strip (or Gap)

The apparent gap in the previous HR diagram for various star clusters seems to correspond to the region where we often find “variable” intensity stars.

Stars moving across the upper region of this strip correspond to Cepheid variables, while stars moving across the lower part of the strip correspond to RR Lyrae variables.

Variable Stars

Variable stars are stars that periodically vary in brightness.

There are two types of variable stars:– RR Lyrae variables correspond to low-mass, post

helium-flash stars. Their periods are all shorter than one day.

– Cepheid variables correspond to high-mass stars and appear to pass back and forth through the instability strip. These stars are particularly important because astronomers have found that their period is directly related to their average luminosity.

Mira (omicron Ceti),the First Variable This pulsating variable was discovered in 1595 by

a Dutch minister and amateur astronomer David Fabricius.

He noted the Omicron Ceti varied in apparent brightness – sometimes being bright enough to see with the naked eye, and sometimes fading completely from view.

By 1660, astronomers realized that the star’s brightness varied with a period of 332 days.

Mira is an example of long period variables – cool red giants that vary in brightness by a factor of 100 or more over a period of months or years.

Mira: A Long-Period Variable

RR Lyra Variables

These are typically low-mass, metal poor stars – often associated with globular clusters.

They all have periods of less than a day.

Cepheid Variables

Cepheids have a characteristic light curve showing an abrupt brightening, followed by a slower dimming.

They are large mass, highly luminous stars which can be seen over great distances.

Delta-Cephei, the first discovered Cepheid variable was discovered in 1784, and was found to vary in brightness by a factor of 2.3 with a period of about 5.4 days.

The Period-Luminosity Plot for Cepheid Variable Stars

Cepheid Variables and Distance Variations in the brightness of Cepheid variables

corresponds to variations in the size of the star (as determined from Doppler shift measurements).

Type I Cepheid variables are metal-rich, and are brighter than type II Cepheid variables.

Cepheid variables can act as “standard candles” to determine the distance to stars, since the period luminosity curve provides a means of calibration of the Cepheids.

Knowing the luminosity, and the apparent magnitude of a Cepheid variable enables astronomers to determine the distance to the star.

The Death of Stars

The manner in which a star “burns out” depends strongly on the mass of the star.

Low-mass stars (with masses less than about 2 – 3 solar masses) end relatively quietly, producing a planetary nebula and a white dwarf.

High-mass stars, however, end much more violently, producing a supernova and either a neutron star, or a black hole – depending upon the original mass of the star.

The Death of a Low-Mass Star

Expansion upon hydrogen burn-out.

Expansion upon helium burn-out.

Planetary Nebula Arise when the Outer Shell of the Star is Blown Away

The shapes of the planetary nebula are quite varied,

depending upon the magnetic fields associated with the star

and upon previous nebula surrounding the star.

White Dwarfs

The remaining core of the low-mass star is small and hot and is called a white dwarf.

This white dwarf slowly burns out, with no further excitement, eventually becoming a black dwarf – unless it has a companion star which can feed it more stellar material!

Companion Stars

White Dwarfs as Companion Stars

Sometimes a white dwarf may be one of a binary star system.

As material (hydrogen) from its companion leaks onto the surface of the white dwarf, the pressure and temperature build up until the outer hydrogen shell ignites and causes an explosion – producing one class of nova.

This type of nova may occur several times, being somewhat periodic as material leaks from the companion star and subsequent nova occur – these are called recurrent nova.

Type Ia Supernova

When the companion star of a binary system expands greatly and looses a large amount of its outer gas shell very rapidly, the white-dwarf companion may be compressed to the point where the carbon-oxygen core begins to “burn”.

This gives rise to a type Ia supernova – a very large explosion which destroys the white dwarf.

The Death of High-Mass Stars

Unlike the low-mass stars, the death of high-mass stars is much more dramatic.

These stars end their existence explosively in what is known as a type II supernova event, where the outer layers of the star are blown away.

During the supernova explosion, the stars luminosity may increase as much as 100 million time.

The inner part of the star is compressed in the supernova explosion and produces either a neutron star, or a black hole.

Pulsars In November of 1967 Jocelyn Bell, working

with a newly developed radio telescope at Cambridge University detected a strange periodic signal with an extremely regular period of 1.3373011 seconds.

This was initial taken as an indication of intelligent life, until other similar objects were detected in other parts of the sky.

The regular pulsing radio sources soon came to be known as pulsars.

Radio Emissions from one of the First Pulsars to be discovered, PSR 0329+54, with a period of 0.714

seconds.

But how can it pulse so rapidly?

What is the source of these Pulsars? Many different explanations were initially proposed,

which were later discarded. Some proposed that the stars surfaces pulsated this rapidly,

but pulsations this rapid would cause the star to explode. Some astronomers proposed that the pulsating signal arose

from the rotation of a star. Although most astronomers at that time believed that the

majority of “dead” stars were white dwarfs, the discovery of very rapid pulsars, like the one in the Crab Nebula (period of 0.033 seconds), indicated that a new type of star – much smaller and much more dense must be the source of these pulsations.

These stars must be similar to the “neutron stars” proposed by Fritz Zwicky and Walter Baade.

Neutron Stars and Pulsars

The Crab Nebula: Remnants of a Supernova X-ray image

The Crab Pulsar Shortly after the discovery of the first pulsars,

strong bursts of radio energy were observed coming from the Crab Nebula with a frequency almost 10 times greater than previous pulsars. This was additional proof that the source must be neutron stars.

Further evidence for a rotating neutron star was the discovery that the frequency of the Crab pulsar was slowly decreasing as would be the case of a source constantly emitting energy.

Later, both an optical pulsar and an x-ray pulsar with the same frequency were observed in the Crab Nebula.

The Spin-Down of a Pulsar

As a pulsars continuously emits radiation, it slowly decreases its rotational velocity. This is the so-called “spin down” of a pulsar.

Adjustments in the surface of a neutron star (similar to earthquakes) cause sudden jumps in the stars angular momentum, introducing “glitches” in a pulsar’s spin-down plot.

Long and Short Pulsars

In 1982, scientists were surprised to find a pulsar with a frequency of 642 Hz. Since then many “millisecond” pulsars have been discovered.

The longest period pulsar has a period of over 8 seconds.

End of Part XI