The Life Cycle of Stars

Star Life Cycle

- This is an area of star development, or where stars are born. - This is a mature star, it also happens to be our sun - This supernova explosion marks the death of a massive star.

The Life Cycle of Stars: Introduction

Before you were born, you lived as an embryo in your mother's womb. You grew and developed until you reached a point where you had to be born in order to keep developing as you should. After your birth, you began progressing from an infant to a youth. You will next grow up and go through various later stages of your life, such as middle age and old age. Stars, likewise, go through life stages. They have different characteristics at each stage, just like you do at each stage of your life. The diagram shows you how one star, our Sun, has gone through various stages since its birth more than 4.5 billion years ago. Right now it is a middle-aged star going through a gradual warming process. Don't worry—it'll be another 10 billion years or so before our Sun enters the old age stage of life and dies. Let's learn how stars form and how they change as they go through various life stages.

Questions 1. How would you compare the brightness and temperature of Betelgeuse with that of Alpha Centauri B? 2. What will happen to the temperature of our Sun when it becomes a red giant? 3. What trend do you notice about star size and brightness? 4. The main sequence stars follow the trend of being brighter when they are hotter. The other groups of stars, however, do not follow this trend. What factor contributes to this?

1. Betelgeuse is slightly warmer and is much brighter. 2. It will be cooler at that stage. 3. The largest stars are the brightest and the smaller stars are dimmer. 4. The giants and super giants are bright, even though t hey are relatively cool, because of their great size; the white dwarfs are dim, even though they are relatively hot, because of their smaller size.

1. Which of the stars is the oldest? 2. Which of the stars is burning hydrogen in the core? 3. Which of the stars is burning helium in the core? 4. Which of the stars has the highest luminosity? 5. Which of the stars has the highest surface temperature? 6. What phase will star c enter next?

1. Which of the stars is the oldest? *star e Feedback: Star e is the oldest because it is a white dwarf. 2. Which of the stars is burning hydrogen in the core? *stars b, c, and d Feedback: Stars b, c, and d are main sequence stars and are burning hydrogen. 3. Which of the stars is burning helium in the core? *star a only Feedback: Star a is a red giant star and is burning helium. 4. Which of the stars has the highest luminosity? *star a Feedback: Star a has the highest luminosity. 5. Which of the stars has the highest surface temperature? *star e Feedback: Star e has the highest surface temperature. 6. What phase will star c enter next? *red giant Feedback: Star c is a main sequence star and will next become a red giant

Check Your Answer

High-mass stars process their hydrogen at faster rates than low-mass stars. Thus, they produce more energy, burn hotter and brighter, and have shorter life spans than smaller stars. So, to answer the question, bigger is NOT better. Smaller stars live longer than larger ones. Eventually, however, every star exhausts its hydrogen supply because the nuclear fusion process keeps going on. As the star gradually converts its hydrogen supply, its core is packed with helium. This causes the core to become increasingly dense and the star's outer layers to cool and expand, which sends the star from early adulthood into middle age.

HR Diagrams Slide 1

In an HR diagram, each star is represented by a dot. There are lots of stars out there, so there are lots of dots. Not every star in the universe is represented on the diagram because that would require trillions of dots! The star's magnitude, or luminosity, is on the vertical axis. It is a measure of the amount of energy the star radiates, or how brightly or dimly it shines. The scale is a relative scale in which star magnitude is compared to the magnitude of a reference star, in this case, our Sun. Our Sun is given a relative magnitude value of one. Other stars are given values higher or lower than one, depending on how bright t hey are in comparison to our Sun.

equilibrium protostar red giant white dwarf supernova nova neutron

- Equilibrium: Our Sun is a(n) _________ star. Our Sun is an equilibrium star. - Protostar: A star in embryo A protostar is a star in embryo. - red giant: A star that burns helium and forms carbon by nuclear reactions A red giant burns helium in its core. - white dwarf: A red giant becomes this after it runs out of fuel A red giant becomes a white dwarf after it runs out of fuel. - supernova: A high-mass star can end its life cycle with a ________ A high-mass star can die with a supernova explosion. - nova: Occurs when a white dwarf adds materials from a nearby red giant A nova occurs when a white dwarf adds material from a nearby red giant. - neutron: A remnant of a supernova is a ______ star. A remnant of a supernova is a neutron star.

Neutron Stars

A neutron star is a remnant produced by a supernova explosion during the death of a massive star. These stars are composed almost entirely of neutrons. They are very hot and are believed to have a heavy liquid interior and solid outer crust. A special type of neutron star, called a pulsar, forms when a neutron star is rotating very rapidly and emits pulses of electromagnetic radiation. Click here to watch a pulsar star send out periodic bursts of radiation

Slide 3

A simple HR diagram is shown here. Notice that our Sun is shown with a relative luminosity value of one. Other stars are either brighter or dimmer than our Sun. Notice too how the color of the graph grades from blue-white to yellow-orange, from left to right. Stars with the highest surface temperatures glow blue-white and those with lower surface temperatures burn yellow-orange. Our Sun is a medium-temperature star that glows yellow.

What process marks the "birth" of a star?

A star is born from a protostar when the protostar becomes hot enough for nuclear fusion in its core to convert hydrogen to helium.

Star Maturity

After its birth, a star continues to be in equilibrium. This means that outward gas pressure from its hot core will be balanced by inward gravitational pressure. The star will continue to convert hydrogen into helium until all of its hydrogen is depleted, which can take millions to billions of years. While it is doing this, it is known as a main sequence star. In comparison to a human, a star in its main sequence stage is one that has reached adulthood. Our Sun is currently in this phase and it converts billions of kilograms of hydrogen to helium every second. It will not run out of hydrogen for about four billion more years. How the star progresses from adulthood to old age depends on its mass. Some stars will live longer than others. How do you think mass determines how long a star lives? Do you think bigger is better? Or do you think smaller stars live longer? Enter your hypothesis and your reasoning behind it in the box below. Then, click through the tabs to check your answer and to learn about what happens to a star after it runs out of hydrogen fuel.

Star Death: High Mass Stars

How do high-mass stars compare to low-mass stars when they come to the end of their life cycle? Click through the tabs to learn about the death of high-mass stars.

Dwarf Stage

Let's look first at how a low-mass star dies. Once all the helium is consumed and converted to carbon, there is not enough mass to increase the star's temperature so that carbon can fuse to heavier elements. Therefore, once the helium is all burned up, the star dies quietly. It ejects its outer layers, which then form what is called a planetary nebula. A planetary nebula is a collection of gases ejected from a low-mass star after its supply of helium is exhausted. The carbon core of the star is left behind and becomes known as a white dwarf. The small star on the right in this image is a white dwarf, and by comparison, it is next to a larger main sequence star on the left. A white dwarf star will slowly lose its heat over time and eventually become a cold dark black dwarf. Note that since the time required for a star to become a black dwarf is longer than the known age of the universe, no black dwarf stars are currently known to exist.

Slide 4

Notice, too, the line of stars that runs diagonally from the top to the bottom of the graph. These stars are main sequence stars, or stars in equilibrium. These stars follow a general trend of being brighter when they are hotter. Stars in this st age are fusing hydrogen in their cores. Red giants and super giants are cooler and brighter than our Sun, whereas white dwarfs are warmer and dimmer than our Sun. Even though a red giant is cooler, it is still brighter to us because of its massive size. White dwarfs are dimmer, not necessarily because of their temperature, which is relatively high, but because of their relatively smaller size.

Star Death: Low Mass Stars

Once they get to the end of middle age, or the end of the helium-burning stage, high-mass and low-mass stars differ in the final stages of their life cycles. Click through the tabs to see how a low-mass star dies.

Supernova

The collapsing material from a high-mass star explodes off of the solid core and sends a shockwave of exploding material through space. This explosion is called a supernova, and it produces a very bright, highly energetic star that can be seen for a few weeks or months. What remains after a supernova explosion is called a supernova remnant. This is the star's outer layers that were blasted into space during the supernova. The gases expand out from the star at incredible speeds and excite the gaseous atoms around it, causing it to glow as a nebula. Depending on the initial condition of the star, what is left will become either a neutron star, a black hole, or it could simply blow itself completely apart, leaving only the remnant.

Dramatic End

The death of a high-mass star is more dramatic. In contrast to a low-mass star, a high-mass star has enough mass to continually increase the temperature of its core and initiate a chain of nuclear reactions. These reactions work by fusion to make heavier and heavier elements from carbon up to iron. Once iron begins to form, the star's core runs out of energy. But it also becomes incredibly dense. The dense inner core of the star sucks in the surrounding layers, making the star implode and collapse in on itself in just a few seconds.

A Summary of the Star Life Cycle

The interactive diagram summarizes the life cycle of a low-mass and a high-mass star. Roll over each part of the diagram to see a summary discussion of what you have read. Be sure to find the following:

Slide 2

The star's surface temperature is on the horizontal axis of the HR diagram. The temperature is usually given in units called Kelvins. You ma y also see HR diagrams that use color on the horizontal axis. This is also correct, since a star's color depends on its surface temperature.

A Star is Born

This massive object in space is appropriately called the Horsehead Nebula. It looks like a horse galloping through the heavens. It is also a place where stars form. Stars form in nebula, or cold dark clouds of dust and gas, like this one. As one of these clouds collapses under its own gravity, it breaks into small pieces. Each of the pieces releases huge amounts of heat energy. Over time, the pieces collapse together to form a rotating sphere of gas. The sphere of gas eventually gets hot enough to resist further collapse. At this point, a protostar forms. A protostar is like a star in embryo—just like a human embryo that has not been born yet. The protostar will continue to grow; adding more and more atoms from the nebular cloud, and increasing in density and mass. Eventually, the protostar will become so hot that hydrogen nuclei in its core will fuse to form helium nuclei. This marks the "birth" of a star. At this point, when hydrogen fusion begins to take place, the embryonic protostar officially becomes a star. It is like the moment when a baby is finally born.

HR Diagrams

Throughout the universe, there are stars in early prototstar stages, dwarf stars, red giants, nova, supernova, and white dwarfs. A Hertzsprung-Russell (HR) diagram is a tool that shows the relationships and differences between stars. The presentation goes over how to interpret an HR diagram.

Section Warm-Up

To us, the stars do not seem to change. We can trace their movements across the night sky in predictable seasonal patterns. We can identify specific stars by their locations and brightness. And on human time scales, stars are unchanging. However, over millions and billions of years, stars go through a life cycle of formation, maturity, and death.

Star Middle Age

When a star begins to burn helium instead of hydrogen, it cools and expands. In this process, helium is converted to carbon. The star begins to glow red and is known as a red giant. If the star is very massive, it may evolve instead into a red supergiant. This is the middle age of the star. This stage of burning helium is the beginning of the end for the star, even though it will go on for several million more years.

Nova

When a white dwarf is close to a red giant, its gravity sucks surface material off the red giant. Large amounts of matter falling into the white dwarf cause instability, and explosions occur in order to release the accumulated material. One of these explosions is called a nova, and the white dwarf star will brighten significantly, as seen from Earth. A nova lasts for about one week, and then slowly dies off; the white dwarf returns to its previous brightness. The image shows a white dwarf accumulating material from a red giant to produce a nova event.