Day 5: Stars

Key moments in history: Launched in 1997 the SOHO observatory has been a bonanza for solar physics. SOHO has now followed our home star though more than one complete 11-year solar cycle, characterizing solar activity at multiple wavelengths. SOHO was on hand to witness several energetic flares, including the Halloween solar storms of 2003.

Key Concepts:

What are stars made of?

Stars are made of very hot gas. This gas is mostly hydrogen and helium, which are the two lightest elements. Stars shine by burning hydrogen into helium in their cores, and later in their lives create heavier elements. Most stars have small amounts of heavier elements like carbon, nitrogen, oxygen and iron, which were created by stars that existed before them. After a star runs out of fuel, it ejects much of its material back into space. New stars are formed from this material. So the material in stars is recycled.

How Long do Stars Live?

The length of a star's life depends on how fast it uses up its nuclear fuel. Our sun, in many ways an average sort of star, has been around for nearly five billion years and has enough fuel to keep going for another five billion years. Almost all stars shine as a result of the nuclear fusion of hydrogen into helium. This takes place within their hot, dense cores where temperatures are as high as 20 million degrees. The rate of energy generation for a star is very sensitive to both temperature and the gravitational compression from its outer layers. These parameters are higher for heavier stars, and the rate of energy generation--and in turn the observed luminosity--goes roughly as the cube of the stellar mass. Heavier stars thus burn their fuel much faster than less massive ones do and are disproportionately brighter. Some will exhaust their available hydrogen within a few million years. On the other hand, the least massive stars that we know are so parsimonious in their fuel consumption that they can live to ages older than that of the universe itself--about 15 billion years. But because they have such low energy output, they are very faint.

When we look up at the stars at night, almost all of the ones we can see are intrinsically more massive and brighter than our sun. Most longer-lasting stars that are fainter than the sun are just too dim to view without telescopic aid. At the end of a star’s life, when the supply of available hydrogen is nearly exhausted, it swells up and brightens. Many stars that are visible to the naked eye are in this stage of their life cycles because this bias brings them preferentially to our attention. They are, on average, a few hundred million years old and slowly coming to the end of their lives. A massive star such as the red Betelgeuse in Orion, in contrast, approaches its demise much more quickly. It has been spending its fuel so extravagantly that it cannot be older than about 10 million years. Within a million years, it is expected to go into complete collapse before probably exploding as a supernova.

Stars are still being born at the present time from dense clouds of dust and gas, but they remain deeply embedded in their placental material and cannot be seen in visible light. The enveloping dust is transparent to infrared radiation, however, so scientists using modern detecting devices can easily locate and study them. In so doing, we hope to learn how planetary systems like our own come together.

Types of Stars:

Protostar: A protostar is what you have before a star forms. A protostar is a collection of gas that has collapsed down from a giant molecular cloud. The protostar phase of stellar evolution lasts about 100,000 years. Over time, gravity and pressure increase, forcing the protostar to collapse down. All of the energy release by the protostar comes only from the heating caused by the gravitational energy – nuclear fusion reactions haven’t started yet.

T Tauri Star: A T Tauri star is stage in a star’s formation and evolution right before it becomes a main sequence star. This phase occurs at the end of the protostar phase, when the gravitational pressure holding the star together is the source of all its energy. T Tauri stars don’t have enough pressure and temperature at their cores to generate nuclear fusion, but they do resemble main sequence stars; they’re about the same temperature but brighter because they’re a larger. T Tauri stars can have large areas of sunspot coverage, and have intense X-ray flares and extremely powerful stellar winds. Stars will remain in the T Tauri stage for about 100 million years.

Main Sequence Star: The majority of all stars in our galaxy, and even the Universe, are main sequence stars. Our Sun is a main sequence star, and so are our nearest neighbors, Sirius and Alpha Centauri A. Main sequence stars can vary in size, mass and brightness, but they’re all doing the same thing: converting hydrogen into helium in their cores, releasing a tremendous amount of energy. A star in the main sequence is in a state of hydrostatic equilibrium. Gravity is pulling the star inward, and the light pressure from all the fusion reactions in the star are pushing outward. The inward and outward forces balance one another out, and the star maintains a spherical shape. Stars in the main sequence will have a size that depends on their mass, which defines the amount of gravity pulling them inward. The lower mass limit for a main sequence star is about 0.08 times the mass of the Sun, or 80 times the mass of Jupiter. This is the minimum amount of gravitational pressure you need to ignite fusion in the core. Stars can theoretically grow to more than 100 times the mass of the Sun.

Red Giant Star: When a star has consumed its stock of hydrogen in its core, fusion stops and the star no longer generates an outward pressure to counteract the inward pressure pulling it together. A shell of hydrogen around the core ignites continuing the life of the star, but causes it to increase in size dramatically. The aging star has become a red giant star, and can be 100 times larger than it was in its main sequence phase. When this hydrogen fuel is used up, further shells of helium and even heavier elements can be consumed in fusion reactions. The red giant phase of a star’s life will only last a few hundred million years before it runs out of fuel completely and becomes a white dwarf.

White Dwarf Star: When a star has completely run out of hydrogen fuel in its core and it lacks the mass to force higher elements into fusion reaction, it becomes a white dwarf star. The outward light pressure from the fusion reaction stops and the star collapses inward under its own gravity. A white dwarf shines because it was a hot star once, but there’s no fusion reactions happening any more. A white dwarf will just cool down until it because the background temperature of the Universe. This process will take hundreds of billions of years, so no white dwarfs have actually cooled down that far yet.

Red Dwarf Star: Red dwarf stars are the most common kind of stars in the Universe. These are main sequence stars but they have such low mass that they’re much cooler than stars like our Sun. They have another advantage. Red dwarf stars are able to keep the hydrogen fuel mixing into their core, and so they can conserve their fuel for much longer than other stars. Astronomers estimate that some red dwarf stars will burn for up to 10 trillion years. The smallest red dwarfs are 0.075 times the mass of the Sun, and they can have a mass of up to half of the Sun.

Neutron Stars: If a star has between 1.35 and 2.1 times the mass of the Sun, it doesn’t form a white dwarf when it dies. Instead, the star dies in a catastrophic supernova explosion, and the remaining core becomes a neutron star. As its name implies, a neutron star is an exotic type of star that is composed entirely of neutrons. This is because the intense gravity of the neutron star crushes protons and electrons together to form neutrons. If stars are even more massive, they will become black holes instead of neutron stars after the supernova goes off.

Supergiant Stars: The largest stars in the Universe are supergiant stars. These are monsters with dozens of times the mass of the Sun. Unlike a relatively stable star like the Sun, supergiants are consuming hydrogen fuel at an enormous rate and will consume all the fuel in their cores within just a few million years. Supergiant stars live fast and die young, detonating as supernovae; completely disintegrating themselves in the process.

Question of the Day:

How many different types of stars are there?

School Activity: Make a Sundial

Activity 1. Making a sundial: Sundials were used to tell the time of the day in olden times and is based on the principle that the position of the sun changes continuously during the day. In reality, it is the Earth that rotates around the sun making it seem like the sun rises in the east and sets in the west. As the sun moves across the sky, the central post on the sundial casts a shadow on its circular plate. It is just like reading a clock – the marks or calibrations on the plate tell you what time it is.

You Will Need:

Step-By-Step Procedure

Making the Sundial:

  1. Using the pencil, poke a hole on the side of the paper / Styrofoam cup approximately 2 inches below its top (rim).
  2. Place the pebbles in the cup so to give it some weight and hold it upright.
  3. Cover the cup with the plastic lid.
  4. Put the straw through the hole on the side of the cup and its lid while letting about half an inch of the straw stick out from the side.
  5. Secure the straw to the cup by taping it down on the side. What you will have now is a cup with a slanting straw passing through it – the major part of it forming an acute angle with the cup’s plastic lid.

Setting up the Sundial:

In order to prepare the sundial you will need to rely on your wrist watch, so do keep it handy.

Begin in the morning on a sunny day when you will be home all day. Identify a spot out in the open that is sunny from 10 AM until 3 PM.

Next, use the compass to locate the North and let the long end of the straw point towards the North. Ensure that the position that you have chosen has no trees or buildings obstructing the Southern direction. Thereafter, place the sundial on level ground or else on a level table.

Once placed, be careful not to move the sundial. It is a good idea to make a mark the original position of the base of the cup on the ground or the table, so that it can be lined up again in case it moves accidentally.

Next, starting at 10 in the morning use the permanent marker to make marks on the lid of the cup where the shadow cast by the straw falls on it. Repeat this process every hour until 3 in the evening. Remember, you can always continue with the marking process in case it becomes cloudy or if you have to leave.

Observation: Simple articles can be used to set up the ancient instrument that was used to tell the time of the day.

Result: Using the wrist watch to calibrate (mark) your home-made sundial, you can use it to tell the time. All that needs to be kept in mind is that it is aligned properly with the straw on the cup’s lid pointing towards the North. - Alernative building a Human Sundial activity

Video of the Day

Youth Ambassador Activity:

Stellar evolution including birth, life and death of stars (potential activity with magnets falling into a gravity well?)

Formation of elements (nucleosynthesis – nuclear fusion)

How to calculate distance to stars (Flashlight activity with standard candles? Knowing luminosity of flashlight and detecting distance to light) Explain HR diagram; Potential to use telescopes, take photos of stars and then place them on HR diagram

Collapse (demo with rubber sheets/balls):

Links: (What is a Star Video) (part 2) (Secrets of the Sun - Nova Video) - your weight on other planets