# Stars and the HR Diagram Dr. Matt Penn National Solar Observatory

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Stars and the HR Diagram Dr. Matt Penn National Solar Observatory

Outline How do we form an HR diagram? Where are most stars? Why?
Absolute brightness (Luminosity) Temperature (Spectral class) Where are most stars? Why? What happens when stars evolve over time? What does an HR diagram of a star cluster tell us?

The basic problem… When we try to understand the life of a star, we face a harder problem than a mosquito trying to understand a human life.

The basic problem… Human life = 3,000 mosquito lives
Stellar life = 100,000,000 human lives We cannot sit back and watch; we need a different approach We use the laws of physics and a few observable quantities to understand the lives of stars

The basic problem… Let’s say the mosquito takes the same approach.
The mosquito wants to measure two things for each person: color of hair, and height of the person. The mosquito makes a graph of these two things and hopes to learn about the lives of people this way.

The basic problem… To measure the height of each person without flying there, what else must the mosquito know?

The HR diagram The tool we use to study stars is called the Hertzsprung-Russell diagram. It plots two observable quantities: the absolute brightness of a star and the temperature of a star. Combined with some laws of physics, the HR diagram provides a way to understand how stars evolve with time.

Absolute magnitude Absolute magnitude: how bright would the star appear if it was 10 parsecs away. To compute the absolute magnitude, we need the apparent magnitude, the distance to the star, and the fact that intensity falls off as (distance)2

Absolute magnitude One way to measure distance is the method of parallax: this is used by surveyors on the Earth’s surface.

Absolute magnitude Another term which is used to represent the true brightness of a star is the luminosity. Once you know the absolute magnitude of a star, you can compute the luminosity, which is often computed in terms of “solar luminosity” by comparing to the Sun.

Temperature To measure the temperature of a star we use must measure the spectrum of the star and then apply more physics We assume the star is a “black body radiator” and then we can compute the temperature from the spectral shape.

Temperature

Temperature

Temperature Looking at individual spectral lines in a star’s spectrum can also reveal the spectral class of the star; spectral class is closely related to the temperature of the star

The HR diagram Most stars lie along the “Main Sequence”
Simple relationship between temperature and luminosity Stars spend most of their lives converting hydrogen to helium, and this is what occurs when the star is on the main sequence An HR diagram of the closest 16,000 stars shows most lie along MS

The HR diagram The HR diagram can be used to determine other parameters of stars, like the radius A black-body radiator has a simple relationship between the absolute brightness (Luminosity) and the temperature L=R2T4, which defines lines of constant stellar radius on the HR diagram.

The HR diagram Stars in the upper right are very large and stars in the lower left are very small. This defines only the SIZE of the star and not the MASS, since the density of stars can be very different. So the branch of stars to the upper right of the MS are giant and supergiant stars.

The HR diagram It is very difficult to measure the mass of a star; it can only be done for binary stars. In a binary system, both objects move around the center of mass of the system, rather than one object “orbiting” the other object.

The HR diagram

Stellar Evolution: 1 solar mass
The “job” of a star is to balance the crushing force of gravity by producing an internal pressure by releasing energy from atomic fusion reactions. When the star can no longer balance gravity, or changes the way it makes internal pressure, we say that the star evolves.

Stellar Evolution: 1 solar mass
Using physics and computer models, we can predict the evolution of stars. The changes which occur in a star even with the same mass as the Sun are profound. Inside, the core of the Sun will run out of hydrogen atoms and eventually turn to helium atoms for energy production.

Stellar Evolution: 1 solar mass
Eventually the Sun can no longer produce internal pressure with fusion reactions; the Sun runs out of energy. The envelope is ejected, and the core of the Sun forms a very dense, solid white dwarf star. A famous planetary nebula with a white dwarf in the center is M57

Stellar Evolution: 1 solar mass
The evolution of a one solar mass star, from the main sequence through the giant phase to a white dwarf, can be traced on a HR diagram.

Stellar Evolution: 1 solar mass

Stellar Evolution: 2 to 5 solar mass
The internal structure, and the evolution of a star varies depending on initial mass

Stellar Evolution: 2 to 5 solar mass
Higher mass stars are much much hotter; they use up their supply of hydrogen much faster than the Sun. A higher mass star can use helium for nuclear fusion, and with the higher temperatures

Stellar Evolution: 2 to 5 solar mass
High mass stars can use heavy elements and can produce nuclei of carbon, oxygen and nitrogen in their core. The nuclei of all carbon, oxygen and nitrogen atoms in the Universe were produced inside the cores of massive stars at earlier times.

Stellar Evolution: 2 to 5 solar mass
When a higher mass star can no longer produce internal pressure, it ejects the envelope in a violent explosion called a supernova. Supernova are so bright they can shine brighter than an entire galaxy, and they can be seen across the visible universe.

Stellar Evolution: 2 to 5 solar mass
Gas thrown off during SN explosions forms remnant nebulae and glows for long times

Stellar Evolution: 2 to 5 solar mass
The collapsing core of a high mass star forms a neutron star, usually in the form of a pulsar, a rapidly rotating stellar remnant which can appear to blink hundreds or thousands of times per second. The most famous pulsar is in the Crab nebula

Stellar Evolution: 5+ solar mass
Stars with initial masses greater than 5 solar masses or so produce violent supernova explosions. The cores of these stars are so massive that they continue collapsing past the neutron star phase and form black holes.

Stellar Evolution: 5+ solar mass
Since black-holes cannot be directly observed, the best support for their existence comes from observations of X-ray binaries. The high temperatures and small size of the X-ray emitters can only be found in the accretion disk surrounding a black hole.

Globular Clusters and HR Diagram
Stars in a globular cluster are all thought to form at roughly the same time. The stars in a globular have different initial masses, and so they will evolve at different rates. If we make an HR diagram of the stars in a cluster, we see stars in various stages of evolution.

Globular Clusters and HR Diagram
By looking at the turn-off point from the Main Sequence, we can estimate the age of the stars in the cluster. Turn-off point  stellar mass  age

Summary Parallax and spectroscopy help us measure the luminosity and temperature of a star. Plotting the luminosity vs temperature gives us an HR diagram. The Main Sequence, where most stars fall in the HR diagram, is where stars convert hydrogen to helium.

Summary We can estimate the radius and the mass of stars based on their position in the HR diagram Evolution of stars occurs as stars run out of fuel and this can be traced on the HR diagram HR diagrams of star clusters help us determine the age of the clusters.

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