The position of the stars (1) The celestial sphere and its terms of reference universal
E ' the surface on which the stars are projected, and serves to represent and study the layout of the various celestial bodies that surround the Earth. Appears to rotate from east to Oves, the Earth is actually rotating in the opposite direction around an ideal axis of the earth whose extension in space touches the side of the North Pole, the celestial sphere at a point near the star Polaris.
To locate the position of a celestial body, you need some reference points that are universal ie independent of the position of observer:
- the 'axis of the world: it is the ideal line is obtained by extending indefinitely the axis of rotation Earth intersects the celestial sphere in two points called the north celestial pole and south celestial pole.
- The celestial equator: it is obtained by extending the floor space of the Earth, that is, that is perpendicular to the axis of the world passing through the center of the celestial sphere. Parallel circles are parallel to the equator in the sky and the celestial meridians are half circles that combine The celestial poles. If the parallel key is the celestial equator, the celestial meridian passes through what is fundamental point range, the point on the celestial sphere where the Sun is the day of the vernal equinox. The point diametrically opposite is the omega point. The omega point range and are on the celestial equator.
(2) Elements of benchmarks on the celestial sphere
are points whose location depends on the position of the observer.
- Zenit is the point where the vertical raised above the head of an observer
meets the sky - the nadir, the point opposite the zenith
- the celestial horizon is the circumference maximum that divides the celestial sphere in an upper and a lower hemisphere. It is obtained by cutting the celestial sphere with a plane perpendicular to the vertical line to the observer.
- The celestial meridian of the place of observation is the maximum circumference that contains the celestial north and south poles, Zenith and Nadir.
(3) Reference elements of the celestial sphere for an observer at the North Pole and the cardinal points
In this case the vertical axis coincides with the Earth and then Zenit coincides with the north celestial pole, the celestial horizon instead coincides with the celestial equator is the great circle described by the apparent motion of stars in their rotation around the axis of the celestial sphere. The stars are not on this vicious circles and instead, called celestial parallels with a diameter gradually decreasing as one approaches the north celestial pole. Polaris will be at Zenit
East (east or east) and West (west or west) are placed in the intersection points between the celestial equator celestial horizon, where the two planes do not coincide (ie if an observer is not place the pole). The local meridian cuts the horizon at two points which are the North and the South posed by part of the pole of the same name. East, West, North and South are the cardinal points.
(4) Determining the position of the stars: the sky coordinates
To place the stars, the Earth is imagined as a reference point and used the celestial equator and the celestial meridian passing through a particular point, the point range of the Constellation of Aries. The point range is the point where the Sun is the spring equinox, March 21. The absolute coordinates, that is independent from the observer's position are:
- the celestial declination: The angular distance between the star and considered the plan of the celestial equator
- right ascension: the angular distance of the star from the meridian Heavenly passing through the point range
altazimuth coordinates are dependent from the point of observation and are
- Height: measured in degrees from 0 to +90 starting from the horizon to the zenith and visible from 0 to -90 at Nadir
- azimuth: measured in degrees along the 'horizon from the south ie from the point where the meridian intersects the local horizon easure growing west. The compass points south, west, north and east that have respectively azimuth 0 °, 90 °, 180 ° and 270 °.
(5) The unit of distance
astronomical units away are mainly three:
- the Astronomical Unit (AU) is the average Earth-Sun distance and amounts to about 149 000 km 600 . It is used within the limits of the solar system
- Year-light (to) is the distance traveled by light in a year that propagates at a speed of approximately 300 000 km / s. In km corresponds to about 9.463 trillion kilometers
- the Parsec (parallax-second, pc): the distance of a point from which an observer would see the semi-major axis of Earth's orbit, perpendicular from the angle of 1 ". This distance is equivalent to 206,265 times the Earth-Sun distance and amounts to 30 900 billion km or 3.26 years-light. The parsec is a measure derived from the method used to determine the distance of stars, which is based on measuring the angle of parallax. The parallax in astronomy is the apparent displacement of a star against a backdrop of distant stars when observed from two points separated by some distance. The parallax is greater the greater the distance between the two different viewpoints, and this is taken as the basic astronomy in the major axis of Earth's orbit, performing two subsequent observations six months apart from each 'other. The characteristics of the stars
(6) apparent magnitude and absolute magnitude
classes magnitude classify stars according to their apparent brightness or absolute. In the first case the brightness of a star is measured with a photoelectric photometer mounted on the telescope. Were, by the ancient Greek Hipparchus, identified six classes of magnitude, we have seen with photometric between a class and the other there is a difference in brightness of about 2.5 times. The classes of magnitude are more than six hours, arriving even negative magnitudes as in the case of the star Sirius (-1.47). You get to perceive images of stars of magnitude 30.
The stellar luminosity that reaches the Earth depends on the actual light emitted by the distance that separates the star from Earth, the classes are for the magnitude of the apparent magnitude (m), the intrinsic brightness of a star is rather indicated by the absolute magnitude (M) that corresponds to the brightness of individual stars that would show if they were placed at a standard distance of 10 parsecs from us. It can be traced back to the absolute magnitude apparent only if you know the distance to the star, however, measured only for a limited number of stars. It may, however, to extrapolate values \u200b\u200bof absolute brightness from the study of stellar spectra.
7) Discuss the information that can be drawn from the study of double stars
The double stars are very close pairs of stars that revolve around their gravitational center of gravity. The study of their orbits (and thanks to the law of universal gravitation) can be traced to their mass, physical parameter otherwise very difficult to determine. These studies showed that most stars have examined mass of between 1 / 10 and 50 times that of the Sun. It 'also possible to derive the diameter of the stars based on the period of concealment of an observer on Earth. Walking around the center of gravity in fact, passing the one before the other, are hidden to each other for a period dependent on their size. They then discovered exceptionally small stars such as neutron stars, as well as the exceptionally large supergiant star with a diameter of more than a thousand times that of the Sun.
8) Describe a stellar spectrum, explaining how you get
Most of the spectra collected from celestial objects is represented by absorption spectra and only in certain situations you have emission spectra. The absorption spectra are obtained when the continuous radiation, coming from the star passes through a low-pressure gaseous substance and less of the hot source, consisting of stellar atmosphere. In this case, the atmospheric gas absorbs some of the radiation emitted by the source, if the radiation is then refracted (prism), we get a colorful spectrum (as a continuous spectrum) interrupted by dark lines or bands corresponding to the absorbed radiation. The number and placement of absorption lines on the spectrum reflect the chemistry of the stellar nature was analyzed to be remarkably uniform in the stars: hydrogen and helium about 80% to 19%, the remainder may comprise all other chemical elements we know.
9) Say how you look at a stellar spectrum in the information that if they draw
The spectrum of the stars is obtained by passing their light through a prism attached to a telescope. It seems a stretch all the colors of the rainbow, from red to blue, interrupted by black lines corresponding to wavelengths missing. The intensity of colors provides background information about the star's surface temperature while the position Black lines informs us about the chemical composition of stars (the outside of the stellar gas). The stars were classified into spectral classes, named by the letters O, B, A, F, G, K, M, reflecting decreasing temperatures. The Sun belongs to the class G that includes yellow stars and surface temperatures of 5000-6000 K. The data instead indicate a prevalence of the chemical composition of hydrogen, about 80%, and approximately 19% helium. The remaining 1% contains all other elements of the periodic table, but not present in all stars.
10) Explain how the Doppler effect applied in the light gives information about the motion of removal approach with respect to Earth or a celestial body
The Doppler effect is a phenomenon that affects all sources of waves, especially for the light that happens when a star is approaching or moving away from an observer on Earth, its spectrum appears with lines respectively move toward the blue (blue shift) or towards the red (red shift). The effect is that in an apparent change in frequency or wavelength of the radiation received. Christian Doppler in 1845, this phenomenon occurred, with the sound waves, firm stance on a railroad track, putting to hear the sound emitted by a car full of musicians: const that the pitch increased when the car was approaching, was decreased when moving away. For the starlight effect of stretching or contraction of wavelengths becomes visible due to the slippage of the spectral lines.
11) What is the space between the stars? There
interstellar matter: very fine dust and gas. This material is not dispersed homogeneously in space but is often found in clusters with dense fog-like appearance and therefore called nebulae. They may or darker than a fund of stars (dark nebulae), or low light because they reflect light and very bright stars very close (nebulae reflection). Only a few can emit a faint light of their own (emission nebulae) the result of a phenomenon of fluorescence produced by the interaction of ultraviolet rays of a few stars near the gas cloud.
The interstellar matter, where it thickens, it can produce new stars.
The evolution of celestial bodies
12) Construction of the HR diagram
E 'was created, independently of each other, the two scientists Hertzprung E. and HN Russell between 1911 and 1913. This is a two-dimensional diagram which highlights the relationship between temperature (abscissa) and absolute luminosity (ordinate) of the stars. Having
data on the visible stars can be seen that their position on the diagram is not random nor are they present in a diffuse way, but instead deal with particular areas. Most are located in a diagonal band, called the main sequence of stars made in which size and temperature are decreasing from top to bottom. The Sun occupies the central region of the sequence. Almost all the remaining stars are arranged in two quite separate areas of the diagram: at the top and right of the sequence is the area of \u200b\u200bvery bright but relatively cool stars (giants and red supergiants), lower left we find very small stars hot (white dwarfs). About 85% of stars belong to the sequence main, only 3-6% to 10% of white dwarfs and the giants and red supergiants.
13) Relation between mass and luminosity of stars
HR diagram relates the brightness temperature without specifying the relationship of these two parameters with the mass of the stars. However, it was later seen that in the brightness of a main sequence star is proportional to its mass: the white-blue stars of the sequence are also those with greater mass, while the red stars, cooler and less bright, are those that have lowest mass. The mass-luminosity relation is not valid for the stars but who are away from the main sequence. For example, White dwarfs are small (small diameter) but have very high mass and therefore a high density. In contrast, the red supergiant star at the top right are bright because they have very large surface, but their mass is relatively small as well so their density.
14) Interpretation of the HR diagram
The HR diagram is a point of reference for understanding all the phenomena of color changes, size and temperature that the stars undergo during their existence, namely to study stellar evolution . The current interpretation of the record is based on the assumption that every region of the diagram is occupied in a stadium can the life of a star, then the denser regions of stars (main sequence) are the evolutionary phase in which a star spends most of its existence. The photographer then the HR diagram stars in different evolutionary stage, captured at the same time. His study led to the conclusion that the position of a star in the diagram is not fixed but depends on factors such as mass, age, chemical composition, which change over time.
In the main sequence, where is the maximum density of points, is the longest and most stable phase in the life of a star. White dwarfs and red giants are different stages of evolution from main sequence stars.
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