Answers to Guiding Questions

Use the questions below as a springboard for discussion. The answers provided are not meant to be exhaustive. Rather, they summarize key concepts introduced in the show where student dialogue may land.

All Grades

What have you learned about stars?

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A star is a huge glowing ball of hot gas, mainly hydrogen and helium. The temperature is so high in its core that nuclear fusion occurs, producing energy. Stars are in equilibrium between the inward force of gravity and the outward force of pressure. Energy created by a star escapes out into space as light of all wavelengths (radio to X-ray), as well as stellar wind. Though stars appear static, they rotate and vary in brightness. They are also all at different points in their lives. There are hundreds of billions of stars in the Milky Way Galaxy alone. Among them is our Sun, the closest star to Earth.

Why are stars important to us?

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Without stars, we wouldn't be here at all. All naturally occurring elements (except for hydrogen, some helium, and trace amounts of lithium) were formed during the life and death of stars. At the end of a star's life, much of its matter is blown into space, where it provides the gas and dust for building new stars and planets.

Closer to home, when our Sun was born, its gravitational force held gas and dust in orbit around it, allowing for Earth's formation.

Now the Sun holds the planets in their orbits, heats the surface of Earth to habitable temperatures, drives Earth's dynamic climate, and fuels photosynthesis.

How do scientists study stars? How do they study the Sun? 

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Scientists can see some nearby stars with the naked eye. But to observe them in detail, we depend on technology on the ground and in space. The majority of our knowledge about stars comes from ground-based telescopes, which enable scientists to see visible light, radio waves, and some infrared light.

Satellites that orbit Earth, orbit the Sun, or journey through space allow scientists to observe light at all wavelengths, free from the blurring and obscuring effects of the Earth's atmosphere, and also enable them to sample the solar wind.

In the lab, scientists conduct experiments to infer atomic and molecular properties of stars, and to investigate how nuclear fusion works.=

Finally, scientists use theoretical modeling and computer simulations to compute how the properties of stars (such as density, pressure, velocity, or composition) change over time. Because much of our data comes from observing light that is already billions of years old, when we study the stars we actually look back in time.

Grades 3-5

What is the Sun?

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The Sun is a middle-aged yellow star of somewhat above average mass. The Sun is the closest star to the Earth, and the Earth travels a complete orbit around it once a year.

How is the Sun important?

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The Sun is a star that nurtures and supports life on Earth. Its heat and light warm Earth's surface, drive dynamic processes such as weather and ocean currents, and fuel photosynthesis. We ourselves experience the Sun's energy every time we feel its warmth on our skin or see with the aid of its light.

How are stars the same? How are they different?

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All stars generate energy deep within their cores by one of the most powerful processes in the Universe: nuclear fusion. Hydrogen nuclei smash together, forming helium and releasing huge amounts of energy. This is why a star shines. The pressure of the gas heated by fusion supports the star against its own gravity.
Stars are different masses, temperatures, and colors. More massive stars are hotter and bluer, while less massive stars are cooler and redder. Yellow stars are in between.

Grades 6-8

How does the Sun affect Earth?

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The Sun's gravitational pull keeps Earth in a steady orbit, which, along with the tilt of Earth's rotational axis, causes a cycle of seasons each year.

The Sun's heat and light warm the Earth's surface, drive dynamic processes such as weather and ocean currents, and fuel photosynthesis. We ourselves experience the Sun's energy every time we feel its warmth on our skin or see with the aid of its light.

The Sun continuously blasts a solar wind made up of charged particles (protons, electrons, and heavier ions). The Earth is almost always protected from the solar wind thanks to our magnetic field, but a trickle of solar wind gets through, sliding down at the poles and producing radiant displays of light called auroras. Magnetic explosions called solar flares produce storms in the solar wind and generate radiation. Under rare conditions, such storms can disrupt radio, cell phones, and GPS, or even cause blackouts on Earth.

How is our Sun similar to or different from other stars?

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The Sun is similar to other stars because all stars create energy in their cores through the process of nuclear fusion. The Sun shines and emits energy at many wavelengths, just like all other middle-aged stars.

Other stars may differ from the Sun in mass, size, temperature, color, age, luminosity, and composition. Many stars have companions in binary or multiple star systems.

What are star clusters?

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Star clusters are groups of hundreds to millions of stars orbiting each other. Like isolated stars, star clusters form from dense clouds of gas and dust. Eventually, mature star clusters eject most of their stars.

What is mass? How does mass relate to gravity?

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Mass is a measure of how much matter or stuff is contained within a given object. The more massive an object is, and the closer one is to it, the stronger the force of gravity towards it. Very massive objects such as stars have strong enough gravity to hold planets and other stars in orbit. Black holes are so small and dense that not even light can escape their gravity.

What are the stages of the life of a star?

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Stars and other celestial objects are born in dense clouds of gas and dust. Gravity pulls the gas and dust into clumps. If the clump is massive enough, a star forms—increased pressure from gravitational collapse raises the temperature, causing nuclear fusion to begin in its core. This heats the core further, raising the pressure high enough to prevent further gravitational collapse. The star remains in equilibrium as long as there is fuel available for fusion. After millions to billions of years, the star runs out of fuel in its core. What happens next depends on the mass of the star.

For low and intermediate mass stars (up to 8 times the mass of the Sun), the outer layers swell enormously and the star turns into a red giant. The star then ejects its outer layers, while the core of the star collapses to form awhite dwarf, which takes billions of years to cool down.

A high mass star (between 8 and 20 times the mass of the Sun) also first becomes a red supergiant and sheds its outer layers. The core of the star collapses violently in on itself, causing the star to explode as a supernova, ejecting even more matter. Its core collapses to form an extremely dense object called a neutron star, which only takes millions of years to cool down.

The most massive stars (over 20 solar masses) form red or yellow supergiants, and then explode in supernovas, forming black holes in their centers. Black holes are so dense that not even light can escape their gravity.

Grades 9-12

What does the Sun emit?

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The Sun emits visible light that reaches Earth and lights our day, and also gives off energy in invisible wavelengths of light, such as gamma rays, X-rays, ultraviolet, infrared, microwave, and radio.

The Sun also emits solar wind: a flow of hot gas that blasts out from the Sun's corona at a million miles an hour.

How do stars form?

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Every star forms in a huge cloud of gas and dust. Over time, gravity causes the cloud to contract, drawing the gas closer and closer together. As more gas accumulates at the center, it becomes denser and pressure increases. This causes it to heat up and begin to glow. A protostar is born. Its gravity continues to pull in gas and dust, further increasing its mass, and thus its pressure and temperature. Eventually, the center reaches millions of degrees Celsius—hot enough to fuse hydrogen nuclei and generate intense energy. The heat generated by nuclear fusion causes the gas at the center of the star to expand, exerting an outward pressure. When hydrostatic equilibrium is reached, a star is born.

Why do stars shine?

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Stars shine because the temperature is so high in their cores that nuclear fusion occurs, producing energy. Radiation and convection carry the energy from the core out to a star's atmosphere. When the energy gets high enough in the atmosphere that the region above it is transparent, it escapes out into space as light of all wavelengths (radio to X-ray), and stellar wind. The escaping energy is the starlight that we see at night, or the sunshine that we see in the day.

What does the color of a star indicate?

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The color of a star depends on its surface temperature. Red stars are coolest, yellow stars intermediate, and blue-white or blue stars are the hottest. The lowest mass, dimmest stars are cool and red, while the highest mass, most luminous stars are hot and blue. The yellow Sun lies in between.

How does life depend on ancient stars?

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Without stars, we wouldn't be here at all. At the beginning of the universe, the only elements that existed were hydrogen, some helium, and trace amounts of lithium. All other naturally occurring elements have been formed during the life and death of stars. At the end of a star's life, much of its matter is blown into space, where it provides gas and dust for building new stars and planets. Even in our own bodies, everything except the hydrogen was formed in ancient stars that lived and died before the birth of the Sun.

How might the Sun impact future stars?

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As a star like our Sun dies, it ejects matter out into space that provides raw materials for building new stars and planets. When our Sun dies, some of the new elements that it has created by fusion will spread throughout our cosmic neighborhood, providing ingredients for new stars, new planets, and perhaps even new life to develop.

How does the discovery of brown dwarfs expand our understanding of stellar objects?

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Brown dwarfs share properties of both stars and planets, having a mass that's in between the two. For every star like our Sun, there are hundreds of brown dwarfs, and similar numbers of low mass stars. Smaller than all stars (less than 8% the mass of the Sun), brown dwarfs have enough mass to generate only limited nuclear fusion, fusing deuterium (heavy hydrogen) for a brief period of millions of years. After this fuel runs out, the brown dwarf simply cools down over billions of years. The discovery of brown dwarfs reminds scientists that the categories that we create to talk about celestial bodies is constantly changing, and that there are many more mysteries to explore in the cosmos!