The filters are named: U ultraviolet , B blue , and V visual, for yellow. These filters transmit light near the wavelengths of nanometers nm , nm, and nm, respectively. The brightness measured through each filter is usually expressed in magnitudes. The difference between any two of these magnitudes—say, between the blue and the visual magnitudes B—V —is called a color index. By agreement among astronomers, the ultraviolet, blue, and visual magnitudes of the UBV system are adjusted to give a color index of 0 to a star with a surface temperature of about 10, K, such as Vega.
Why use a color index if it ultimately implies temperature? Because the brightness of a star through a filter is what astronomers actually measure, and we are always more comfortable when our statements have to do with measurable quantities.
Stars have different colors, which are indicators of temperature. The hottest stars tend to appear blue or blue-white, whereas the coolest stars are red. A color index of a star is the difference in the magnitudes measured at any two wavelengths and is one way that astronomers measure and express the temperature of stars. The star would therefore appear white — a combination of all colors. Earth's sun emits a lot of green light, but humans see it as white. Purple stars are something the human eye won't easily see because our eyes are more sensitive to blue light.
Since a star emitting purple light also sends out blue light — the two colors are next to one another on the visible light spectrum — the human eye primarily picks up the blue light. Recall from Lesson 3 that the spectrum of a star is not a true blackbody spectrum because of the presence of absorption lines. The absorption lines visible in the spectra of different stars are different, and we can classify stars into different groups based on the appearance of their spectral lines.
In the early s, an astronomer named Annie Jump Cannon took photographic spectra of hundreds of thousands of stars and began to classify them based on their spectral lines.
Originally, she started out using the letters of the alphabet to designate different classes of stars A, B, C…. However, some classes were eventually merged with others, and not all letters were used. The original classification scheme used the strength of the lines of hydrogen to order the spectral types.
That is, spectral type A had the strongest lines, B slightly weaker than A, C slightly weaker than B, and so on. For more information on her life and work, visit the homepage for Annie Jump Cannon at Wellesley College.
Recall from Lesson 3 that the electrons in a gas are the cause of absorption lines—all the photons with the correct amount of energy to cause an electron to jump from one energy level to a higher energy level get absorbed as they pass through the gas. The absorption lines from hydrogen observed in the visible part of the spectrum are called the Balmer series , and they arise when the electron in a hydrogen atom jumps from level 2 to level 3, level 2 to level 4, level 2 to level 5, and so on.
The strength of the Balmer lines that is, how much absorption they cause depends on the temperature of the cloud. If the cloud is too hot, the electrons in hydrogen have absorbed so much energy that they can break free from the atom. So, very hot stars will have weak Balmer series hydrogen lines because most of their hydrogen has been ionized.
Carbon stars were traditionally classified as R and N classes with similar temperatures to K and M stars respectively. Nowadays they are collectively referred to as type C for Carbon. In order to use the colour of a star astronomers first need to define it and then have a way to measure it.
Luckily there is a simple way to do both that relates back to the spectrum of a star. A typical star's spectrum approximates a Planck curve. This means the intensity of emitted energy varies with wavelength such that a hot star emits relatively more energy at blue wavelengths than at red whilst a cool star's emission peaks at red wavelengths.
If the intensity is measured at a specific wavelength or narrow waveband then it can be compared with intensity at other narrow wavebands. The intensity is expressed as apparent magnitudes.
Rather than just have one apparent magnitude, m measured across the entire visible spectrum we can use a filter to restrict the incoming light to a narrow waveband.
If, for instance, we use a filter that only allows light in the blue part of the spectrum, we can measure a star's blue apparent magnitude, m B. This is generally abbreviated to B. A blue filter provides a waveband similar to the maximum sensitivity of most photographic film which peaks at blue wavelengths of around nm which is why red lights are used in darkrooms.
Similarly if we use a filter that approximates the eye's visual response which peaks in the yellow-green part of the spectrum we measure m V or V for a star. This system of measuring magnitudes at two different wavebands, B and V forms the basis of defining the "colour" of stars. Rather than use words such as red or orange, astronomers define the colour of a star to be its colour index. Colour index or CI is simply a number equal to the difference between the blue, B and visual, V magnitudes of a star.
This is shown by equation 4. How does this apply in practice to stars? Let us look at two different stars, one with an effective temperature of 15, K and the other of 3, K. Each of these will produce a spectrum that approximates a black body curve. The diagram below shows these two curves on a normalised intensity plot if we used a true intensity scale the plot for the 3, K star would be dwarfed by that for the hotter star.
You can see that in the visible part of the spectrum the curve is sloping down to the right for the 15, K star whilst it slopes up for the cooler star.
The two black lines represent the peak wavelengths for the B and V filters. The blue curve represents the 15, K star. It emits more energy in the B waveband than in the V waveband. This means that it is brighter in B than in V therefore its apparent magnitude B will be lower than apparent magnitude V.
This is shown in the diagram below:. The diagram below shows this:. The calibration of the colour index scale means that a star of spectral class A0 and luminosity class V ie a main sequence star has a colour index of 0. Stars hotter than Vega will have a negative colour index and appear more bluish. Stars with a positive colour index are cooler than Vega and will appear more yellow, orange or red. Note the "shade" of colour is an indistinct term whereas the colour index is a directly measurable value.
A range of colour indexes related to spectral class are shown in table 4. Colour Index ranges from about Luminosity class does affect the colour index for a star so that a main sequence V star and a supergiant Ia of the same spectral class may not have have quite the same CI but this correction factor is beyond the scope of the syllabus.
A table showing the colour index for main sequence luminosity class V stars is below:.
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