12.1 - Space Flashcards
Stars are at such enormous distances from the earth, what information can we get from them???
The only information we have about them is the electromagnetic radiation we receive from them.
We know the electromagnetic radiation we receive from stars on earth, what can this be used for
From this limited information, we can measure various stellar properties. These allow us to classify stars into various groups which have quite enigmatic names, such as red giant, white dwarf and blue supergiant. They are much too far away for us to send probes to them, or even to send signals to them in the hope of detecting reflections.
However, the electromagnetic emissions from stars can tell us their temperature, chemical composition, speed of movement, approximate age, size and much more! 🤩
Is there a better method to determine how bright a star is other than just observing it with the naked eye?
With the naked eye, we are only able to distinguish six different levels of how bright stars appear to us. This is insufficient for scientific use, as many stars of differing brightness would appear identical to our eyes.
Astronomers therefore use a more precise measure to classify the actual brightness of stars: their output power, which is known as luminosity.
What can luminosity be defined as
We define luminosity as the rate at which energy of all types is radiated by an object in all directions. This depends upon both objects size and it’s temperature
What does an object/stars luminosity depend on
The objects size and more importantly, it’s temperature
Explain the black body radiation curve for different temperatures
Y axis = energy output
X axis = wavelength
The higher the temperature of the star, its energy out put peak will be further to the left and a higher peak overall.
The distribution is given by the Stefan-Boltzmann law. This tells us that the output power from a black body is proportional to its surface area and the fourth power of temperature.
Since L = sigma x A x T^4
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What’s a perfect black body radiator
A perfect black body radiator will give off energy across the entire EM spectrum
What is the Stefan-Boltzmann law
L = sigma x A x T^4
Where L = luminosity
A = surface area
T = temperature in kelvin (SI unit)
The Stefan-Boltzmann constant, sigma is 5.67 x 10^-8 W m^-2 K^-4
For a spherical object this equation would become
L = 4 x pi x r^2 x sigma x T^4
Working on the assumption that a star acts like a black body emitter, which is a very good approximation, this equation describes the luminosity of a star.
What is important to remember about black body radiators
A black body radiator is a theoretical perfect emitter, which follows the Stefan-Boltzmann energy output curve for its temperature and also follows Wien’s law.
Remember black body radiation is a thermodynamics idea that can be applied in other areas of physics.
What is the range of wavelengths emitted by a star also known as
It’s spectrum
How can we determine the temperatures of stars
There are various methods, but we will focus on one that uses the wavelengths of light given off by a star
When we examine a stars spectrum, we find that some wavelengths are given off with more intensity than others.
Tell me about Wien’s law
We saw from the Stefan-Boltzmann law that as the temperature of a black body increases, it emits more energy. At higher temperatures the black body radiation curve has a more pronounced peak, and the wavelength of the peak output gets shorter as the temperature rises. The relationship between the peak output wavelength and temperature is described by Wien’s law
Lamder(subscript max)x T = 2.898 x 10^-3 mK
The number 2.898 x 10^-3 m K is known as Wien’s constant
Define red giant
A red giant is a large star, somewhat cooler than our sun, eg 3000 K
Define a white dwarf
A white dwarf is a small hot star, perhaps 10000 K
Define blue supergiant
A blue supergiant is a very large, very hot star, perhaps 25000 k
Define luminosity
Luminosity is the rate at which energy of all types is radiated by an object In all directions
Define Stefan-Boltzmann law
The law is that the power output from a black body is proportional to its surface area and the fourth power of its temperature in kelvin
L = sigma x A x T^4
Define Wien’s law
Wien’s law is that the relationship between the peak output wavelength and temperature for a black body radiator is given by the equation:
Wavelength(subscript max) x T = 2.898 x 10^-3 mK
Tell me about star classes
Astronomers have classified stars into groups according to their temperature. This is a useful property to use since stars with similar temperatures tend to share many other features. The temperature determines the spectral output of the star, but it can also suggest chemical composition and age.
What are the spectral classes and how can you remember them
From hottest to coldest O B A F G K M
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Tell me about trends in spectral classes
The hotter stars tend to have more mass and are more luminous. The larger gravitational pressure at the centre of a massive star makes the nuclear fusion reactions within the star run very fast, producing a lot of energy and using the hydrogen fuel in the star at an incredible rate. In addition, more massive (as in higher mass) are also larger.
Therefore, with large size and high temperatures, the hotter stars are very luminous, giving off a great deal of energy. At the same distance away from us, these will then appear very bright in the night sky compared with a smaller, cooler star.
Tell me about changes in colour between spectral classes
The overall impression of the O class spectrum is a bluish colour, whereas for the M class spectrum there is a pronounced red colour showing. These changes are a direct result of the variation in output curve we saw associated with Wien’s law.
The peak of the black body radiators wavelength output - will show the colour the star appears.
Wien’s law curve shows us what colour a star will appear.
Why is the Hertzsprung-Russel diagram called a diagram and not a graph
If you were to plot a graph of luminosity against start temperature, you can confirm this trend. There is a general correlation.
However, the more data you add, the more complex the picture becomes. We must also remember that the temperature measurement assumes the star behaves as a black body, and the luminosity is similarly often not as accurate as we would like. In general, luminosity has to be determined from a calculation that includes the distance to the star, and measuring the distances to stars is by no means an exact science.
Such a lot can give us some very useful insights, but it’s not a graph In the true sense. It is known as a Hertzsprung-Russel diagram
What is the Hertzsprung-Russel diagram like
Most stars we observe fall on a diagonal line across the Hertzsprung-Russel diagram, which is called the main sequence. These are stable stars which will exist in this state for the majority of their lifetime. Their correlation represents the connection between brightness and high temperature.
Note that the plot is ALWAYS drawn with hotter temperatures on the left hand side !
There are also other stages in a stars evolution, which appear in other places on the diagram, but these are much shorter than it’s stable period. Thus, there are far fewer stars in those parts of the diagram.