365daysDate: August 18, 2009

Title: Helium and the Sun


Podcasters: Susan and Amanda Murph

Organization: How to Grow Your Geek Podcast

Description: In honor of the anniversary of the discovery of Helium, Susan Murph and her 9 year old daughter, Amanda, give us a quick history lesson on what we know about that element and our solar system’s star, the Sun.

Bio: Susan is also a life-long sci-fi, fantasy and science geek, and loves to incorporate her favorite hobbies into her current career of raising her two kids as a stay-at-home mom. She believes that including her kids in her hobbies not only strengthens her relationship with them, but also benefits their development of useful skills such as critical thinking, logic, creativity and reading comprehension, just to name a few. Susan currently hosts and produces the “How to Grow Your Geek” podcast.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by John wants McKenna, Johnnie, and Logan to know that he loves you tons, and to everybody else, space is just as cool as it was when you were kids.


Susan: Hi, my name is Susan Murph, and I am the host of How to Grow Your Geek: Parenting and More! My daughter Amanda and I enjoy studying space and astronomy, and today, in honor of the anniversary of the discovery of the element Helium, we are going to tell you some of what we know about our Sun and the Helium!

So, let’s get started. Amanda, tell me what you know about the size of our Sun.

Amanda: The Sun is by far the largest object in our solar system!

Susan: It contains more than 99.8% of the total mass of the Solar System – Jupiter contains most of the rest. Our Sun is an “ordinary” star, in that there are a lot of stars out there like ours. But there are many more smaller stars than larger ones, and ours fits into the larger category based on mass. So, our sun is “ordinary”, but bigger than most stars.

Amanda: So, if our sun is so big, why does it look small, almost the same size as the moon?

Susan: It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth’s orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slightly imperfect then the Moon covers only part of the Sun’s disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long.)

Amanda: How often are there eclipses of the Sun? Can I see one?

Susan: Eclipses of the Sun happen once or twice a year. If you stay home, you’re likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel half way around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it’s time to sleep. And you can see the solar corona. It is well worth a major journey.

Amanda: Now, what is the Sun made of?

Susan: The Sun is, at present, about 70% Hydrogen and 28% Helium by mass. Everything else, called “metals” in astronomy, amounts to less than 2%. This changes slowly over time as the Sun converts Hydrogen to Helium in its core. The conditions at the Sun’s core which is the inner 25% of its radius, are really extreme. The temperature is 15.6 million Kelvin and the center of the core the Sun’s density is more than 150 times that of water.

Amanda: How does the Sun convert Hydrogen into Helium?

Susan: The Sun’s power is produced by nuclear fusion reactions in the core. Each second about 700,000,000 tons of Hydrogen are converted to about 695,000,000 tons of Helium and 5,000,000 tons of energy in the form of Gamma Rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light.

Amanda: It is this energy, mostly visible light, that reaches us here on Earth, and enables us to live here, by allowing plants to grow and animals to gain energy from the plants.

Susan: In addition to heat and light, the Sun also emits a low density stream of charged particles, made up of mostly electrons and protons, known as the solar wind which propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful Aurora Borealis, or Northern Lights. These swirly patterns of colored light can often be seen in the skies over North America, especially in Alaska and Canada.

Amanda: The sun gives off wind? Does that make it harder to travel in space?

Susan: The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft. There are even theoretical plans for a Solar Sailboat, with huge sails that are pushed by the solar wind as the means for traveling through space.

Amanda: So, how old is our Sun?

Susan: Don’t worry, the Sun appears to have been active for 4.6 billion years and has enough fuel to go on for another five billion years or so. Since its birth it has used up about half of the Hydrogen in its core. It will continue to radiate “peacefully” for another 5 billion years or so (although its luminosity will approximately double in that time.) But eventually it will run out of Hydrogen fuel.

Amanda: So what happens when it runs out of Hydrogen fuel?

Susan: At the end of its life, the Sun will start to fuse Helium into heavier elements and begin to swell up, ultimately growing so large that it will swallow the Earth. After a billion years as a Red Giant, it will suddenly collapse into a White Dwarf — the final end product of a star like ours. It may take a trillion years to cool off completely.

Amanda: Well, that is a bummer. Hey, what is this Helium stuff anyway?

Susan: Helium is the chemical element with atomic number 2, and is represented by the symbol He. Helium is the second lightest element and is the second most abundant in the observable universe, being present in in the Universe in masses more than 12 times those of all the other elements heavier than Helium combined. In our own Sun and Jupiter, Helium is present in this proportion.

Amanda: Why is there so much Helium in the Universe?

Susan: This high abundance is due to the very high binding energy (per nucleon) of Helium-4 with respect to the next three elements after Helium (Lithium, Beryllium, and Boron.) This Helium-4 binding energy also accounts for its commonality as a product in both nuclear fusion and radioactive decay. Most Helium in the Universe is Helium-4, and was formed during the Big Bang. Some new Helium is being created presently as a result of the nuclear fusion of Hydrogen in all but the very heaviest stars which fuse Helium into heavier elements at the extreme ends of their lives.

Amanda: How do we know that Helium is so common?

Susan: Using a technology called spectroscopy, we can look at the light emitted from a star or a planet, and tell, by measuring the frequencies and wavelengths of the light, what elements make up the object we are looking at. Most telescopes are equipped with a mass spectrometer, used to measure the chemical composition and physical properties of astronomical objects. So we can look for the wavelength and frequency of Helium, and we can see that it is really very common in the Universe, because it occurs in nearly every object we observe. It is good thing we can identify it this way, since Helium is a colorless, odorless, tasteless, non-toxic, inert gas. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions.

Amanda: So, if it is odorless, colorless, and tasteless, then how did we discover Helium?

Susan: Today is the anniversary of the discovery of Helium! The first evidence of Helium was observed on August 18, 1868, as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Pierre Janssen while he was observing a total solar eclipse in India. This line was initially assumed to be Sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum. He concluded that it was caused by an element in the Sun unknown on Earth. Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, Helios.

Amanda: Ok, so I know how Helium is used in the Sun, so what is it used for on Earth?

Susan: We have many uses for Helium here on earth. One of the most common uses is filling party and weather balloons as it is non-flammable. It also makes a wonderful cooling medium for nuclear reactors, since Helium won’t become radioactive. Helium is also used for isotopic dating by Helium ratios for substances such as seawater, ocean beds, to estimate age. It is a key ingredient in heart surgery as part of a Helium cardio-pulmonary resuscitation pump. And, it can be used as a gas for supersonic wind tunnels, as well as creating Helium/Neon lasers and making orange “Neon” signs.

Amanda: I knew Helium balloons were special!

Susan: So now you know a little of how our Sun works, and why Helium is important. The next time you have a Helium balloon, know that you are holding a little bit of the most abundant element in the Universe!

Amanda: We hope you have enjoyed our presentation, and if you would like to hear more from us, please visit

Susan: Thanks so much, and please have a great Year of Astronomy!

End of podcast:

365 Days of Astronomy
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