PLEASE, THE ANSWER MUST BE TYPED!! Comment on the Stellar Spectra, OBAFGKM and h

Question

PLEASE, THE ANSWER MUST BE TYPED!!

Comment on the Stellar Spectra, OBAFGKM and how it's related to the temperature of stars. What do you find interesting or hard to understand in this section? Recently Astronomers have added to this pneumonic new category of stars, what are the new letters they have added and why? Comment on the contribution of the woman astronomers at Harvard led by Annie Jump Cannon; what role did they play in our understanding of the universe and the establishment of the field of Astrophysics?? Do you understand why different stars have more well-defined absorption lines of different elements based on their temperatures?? Which type of stars gives the thickest and most well defined Hydrogen Balmer lines? How about the B-V Color Code of stars, does that section make sense or do you have any questions about it? Now, comment on the HR diagram and how it functions and how it helps us put everything together. Do you understand the HR diagram and how it works, if not what questions do you have? What do you find interesting or new about the HR diagram that intrigues you? What do the different sections of the diagram mean like Roman Numeral 5, 4, 3, 2, 1a, 1b? What kind of stars go in the different corners of the diagram like lower right, upper right, etc?? What are the most abundant stars in the universe and the least abundant stars?

Explanation / Answer

Stars are currently classified under the Morgan – Keenan (MK) system, based on their spectral characters. There are seven categories – O, B, A, F,G, K,M, each of this is subdivided further into ten categories. Principally based on the pattern of spectral lines, formed due to electromagnetic radiation from the star.As the strength of the spectral lines vary with surface temperature of the star, this classification scheme also reflect different temperatures of the stars. While O denotes the hottest, M denotes the coolest. Within each letter class there are ten subdivisions, denoted by numbers 0(hottest) to 9(coolest). Close observation of the figure reveals that there are very few spectral lines in high temperature classes like O, B, which in turn also shows simplified atomic structure in high temperatures and vice – versa.

Luminosity classes are added later to this category. Along with temperature, this classes consider the luminosity, which is related to the specific gravity or density of the stars. As denser stars with higher surface gravity exhibit greater pressure broadening of the spectral lines, difference in spectral character thus also reflect difference in luminosity, thus differenciate giant and dwarf stars. Luminosity class 0 or Ia+ is used for hypergiants, class I for supergiants, class II for bright giants, class III for regular giants, class IV for sub-giants, class V for main-sequence stars, class sd (or VI) for sub-dwarfs, and class D (or VII) for white dwarfs.

Astronomers led by Annie Jamp Cannon, had great contributions in studying the stellar spectra, early in the 20th century. With Edward C. Pickering, she is credited with the creation of the Harvard Classification Scheme, which was the first serious attempt to organize and classify stars based on their temperatures and spectral types. Her women group ‘Pickering,s Women” had immense contribution in studying, mapping, identifying every individual star in the sky. She manually classified more stars in a lifetime than anyone else, with a total of around 350,000 stars. She discovered 300 variable stars, five novas, and one spectroscopic binary. She discovered her first star in 1898, though she was not able to confirm it until 1905. Based on Balmer absorption lines, they classified stars into the OBAFGKM scheme, which is valid even today.

Spectral lines are dark or bright lines, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are very atom – specific, thus gives accurate information about the atomic structure, chemical composition of the stars. Light moving outward through the high density interiors of stars produce a continuous soectrums, while light reaching low density region of solar system, some lights are absorbed. So, this differences in absorption, reflection of energy levels results into the absorption spectrum of different elements.

A type of stars have the strongest hydrogen lines the series.

In astronomy, sometimes very simple numerical symbols are used in determining the color of objects. It is regarded that hot stars(low B-V) are blue or purple, larger the color index (B-V), more red and cool the stars are. B is sensitive to blue light, and V is sensitive to visible (green-yellow) light. I don’t think the scheme is completely reliable, befor confirming the temperature of the stars, along with the color, other factors must also be considered.

HR is a graphical rool where astronomers have put all important characters of stars like temperature, luminosity, spectral type, color and evolutionary range together. It is fascinating to find out how the stellar system has evolved. This tool has very significant role in every advancement of astronomy, modern physics. this scatter plot of stars shows the relationship between the stars' absolute magnitudes or luminosities vs their stellar classifications or effective temperatures.

I have described what the roman numbers mean in the diagram and also what kind of stars are present in different corners in the plot (Please refer to diagrams, you will easily find them over internet)

I (Supergiants), II, III(Giants), IV(Subgiants), V(Dwarfs)

Upper left - Blue giants, Upper right – Red supergiants.

Middle left and lower right – Main Sequence, Middle right – Red giants.

Lower left – white dwarfs.

Red dwarf or M type stars are the most abundant stars in the universe.

Blue giants are the least abundant stars in the universe.

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