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---------------------- 2278 - Astronomer’s most Famous Diagram
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- One of the most famous diagrams and most useful in astronomy is the Hertzsprung -Russell Diagram. ( HR Diagram ). It was created from a simple idea. Plot the characteristics of stars in brightness versus their color. Brightness is a measure of luminosity, or intensity of radiation. Color is the surface temperature of the star.
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- Published in 1908 and in 1911 the HR Diagram reveals the lifetime of stars. Each star has a predetermined lifetime and evolution depending on its mass. The diagram successfully grouped the stars according to their stage in life.
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- The plot displayed a distinct diagonal line of dots (stars ) from the upper right (hot and blue stars ) to the lower left ( cold and red stars ). The diagram plotted descending temperatures on the x-axis from 31,000 Kelvin (blue) to 2,200 Kelvin (red) And, plotted ascending brightness on the y-axis from 1/100,000 Sun’s brightness to 100,000 times the Sun’s brightness.
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- The diagonal line became known as the “ Main Sequence” line for star evolution. All stars generate electromagnetic energy, the light spectrum from radio waves to Gamma Rays. All stars do this be converting Hydrogen into Helium in nuclear fusion at the core of the star.
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- The more massive the star, the stronger the gravity, and the hotter the core, the faster the nuclear reaction, the brighter the star, and the shorter its lifetime.
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- There are some groups of stars that are off this Main Sequence diagonal line. The stars in the upper right corner are stars that are cooler yet brighter than our Sun. Our Sun is on the Main Sequence line and about in the middle at 6000 Kelvin.
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- How can a cooler star be brighter, 100 to 1,000 times brighter than our Sun?
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- Answer: These stars are enormous. Supergiants. They are so big they produce less light for every square inch of their surface.
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- There is another group of stars off the diagonal line in the lower left corner that are dimmer and hotter. These stars are tiny. The are called White Dwarfs. White hot and tiny. This is what our Sun will be in 5,000,000,000 years from now.
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- The Pleiades Cluster is a group of stars that include bright blue stars. The cluster is in the Constellation Taurus the Bull and is often called the “ Seven Sisters”. In Japan it is called “ Subaru”. The Hyades Cluster , also in the Constellation Taurus, is missing blue stars. Therefore, the Hyades Cluster must be older stars. The bright blue stars in the Hyades Cluster have all exploded as supernovae and died off.
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- The bottom-right of the diagonal line are the dimmest, reddest, and least massive stars. the smallest is about 8% the mass of the Sun. These are called Brown Dwarfs and are too lightweight to sustain nuclear fusion. They are 1/100,000 the Sun’s brightness and 3,000 Kelvin.
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- The upper-left of the diagonal line are the brightest, hottest and most massive stars. Up to 100,000 the Sun’s brightness and 30,000 Kelvin. The bright stars are the rarest because they do not live long, maybe only a few million years before they explode in a supernova.
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- The largest star might be 150 times the Sun’s mass. However, some astronomers claim they have found stars up to 320 times the Sun’s mass. The very first stars in the Universe may have been this massive.
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- Early evolution stars were made of only Hydrogen and Helium. Second and third generation stars contain Carbon and Oxygen that were created in the earlier supernovae explosions. Carbon and Oxygen emit infrared light (heat energy). This allows them to cool faster.
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- Today’s stars are second and third generation stars and likely not as massive , only 100 times the Sun’s mass. Any star that is greater than 8 Solar Mass will end its life exploding as a supernova. Smaller stars like our Sun will die as Planetary Nebulae and then White Dwarfs. They will not explode as supernovae.
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- In our Milky Way Galaxy, out to 20,000 lightyears away, there are only a few stars that might explode as supernovae. 1604 was the year of the last one recorded. It is now know as Kepler’s Supernova.
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- Betelgeuse and Antares are the nearest and brightest stars likely to explode next. They are 640 and 550 lightyears away respectively. When they explode they will be as bright as the Full Moon in the sky, but, they are far enough away that the explosion itself will not reach us.
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- The 1987 Supernova explosion occurred in our sister galaxy, the Large Megellanic Cloud galaxy, But, it was not a red super giant star, it was a blue star that went supernova. Two stars that are similar in this way in our galaxy are Deneb and Rigel.
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- Other types of supernovae come from binary stars. One star steals mass from its orbiting companion star until it reaches 1.4 Solar Mass and explodes. Unfortunately these exploding White Dwarfs are so dim before they go supernova it is hard to predict how many are out there as binary stars. Almost 60% of all stars are binaries.
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- Can you believe all this was learned from a simple diagram invented in 1908. There is still more to learn:
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- In this case the HR Diagram is used to determine the age of a Globular Cluster of stars. We can assume the stars formed at about the same time and are all the same distance away. The distance was measured to be 7,800 lightyears away.
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- All the stars in the cluster do not have the same mass. The mass of each star determines its longevity. The more massive stars are the color blue, have higher luminosity and have the shortest lives. The smaller stars are red in color , have the lowest luminosity and live the longest.
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- After plotting the temperature, which is the color, versus the luminosity, which is the intrinsic brightness, we see that the HR Diagram comes to diagonal and a main sequence turnoff. The diagram shows that the blue stars run our of fuel first.
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- When a star runs out of fuel , or hydrogen burning, it begins fusing helium. Because this happens at a much higher temperature the star expands into a Red Giant. The blue star has evolved into a bright red star. These main sequence stars trace a diagonal line that ends with a horizontal line called the red giant branch and the top of the branch disappears. The stars die.
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- This end of life is the turnoff on the diagram and it can be used to determine the age of the cluster of stars. A bright O-star will live 1 million years. A G-type star, like our Sun, will live 8 billion years. A faint, red M-star will live 56 billion years.
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- This cluster being studied , NGC6397, contains 400,000 stars. The color and magnitude of each star plotted on the HR Diagram identifies the main sequence turnoff. This point in turn can be used to determine the age of the cluster to be 13.4 billion years.
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- The light we receive is 7,800 years old which is only 0.000,058 percent of the age of the cluster. 7,800 years is how long it took the light to reach us. And, the HR diagram told us the age of the cluster. This diagram has taught us a lot about stellar evolution. Stay tuned, there is a lot more to learn.
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- February 17, 2019 1283
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-------------------------- Monday, February 18, 2019 --------------------------
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