OUR STAR - THE SUN
The image above shows the life cycle of our sun. Our sun is a star, just life many (billions) of other stars. There are two ways to measure stars -- their apparent brightness and by star type. Brightness is measured by "magnitude". This is a list of the brightest 150-odd stars in the sky showing the different types of stars that exist in the universe. As you can see, our star - the Sun - is only just entering middle-age. It will be another 5 billion years before the Sun becomes a Red Giant.
First though, what is a star? A star (like our sun) is like a huge nuclear reactor. It's really gas that has coalesced into something so heavy, its gravity not only holds it together, but creates a tremendous amount of heat and that heat is radiated. Think of the star as a giant nuclear pressure cooker.
Stars like everything, from atoms to galaxies, have life cycles. There are young stars, middle-age stars (our sun is an early middle age star), and old ones. Unlike human beings, star lives are measured in billions of years (so far as we know). If you look at different parts of the sky using a telescope, you can see stars of all ages.
This is a star nursery. It is a planetary nebula that was probably born following the death (explosion) of an old star. Inside the nursery, you can see thousands of new stars are being born.
Here, we see a dying star that has exploded in front of our very eyes. The discovery was made by the renowned Australian amateur supernova hunter Reverend Robert Evans, while simply scanning galaxies with a 12-inch (31-cm) backyard telescope from his home in New South Wales, Australia in June 2003). Following Evans' discovery (identified as SN2003gd), the Gemini team (Gemini is one of the telescope arrays on Mauna Kea in Hawaii), quickly followed up with detailed observations using their equipment and the Hubble Space Telescope. These observations verified the exact position of the original or "progenitor" star. Using this positional data, the team dug through data archives and discovered that observations by the Gemini Observatory and HST contained the combination of data necessary to reveal the nature of the progenitor.
Our sun does not have the mass -- it is not big enough -- to become a Supernova, but when it gets old, it will grow into a Red Giant, and consume all of the inner planets: Mercury, Venus, Earth and Mars. Then it will collapse into a tiny star called a white dwarf. Nevertheless, our sun is extremely powerful, and its radiation (solar winds) extend to the edge of our Solar System.
The Japanese Hinode spacecraft took this video of our star/sun showing it giving off giant flares that are larger than Earth. Sometimes, these erupt into "Coronal Mass Events". These "Coronal Mass Ejections" or CMEs radiate all over the place, and the winds that the sun create with its pressure and gas releases, extend to the farthest reaches of our solar system. When the solar radiation hit's Earth's atmosphere, it generally bounces off of the atmosphere or off of thick clouds. Some radiation gets through, and warms our planet and gives us light. CME's, when directed at Earth, can disrupt communications satellites, and even destroy some spacecraft. We usually have some notice of CME events by observing closely, solar activity. When there is a threat, astronauts in the Space Station must protect themselves inside the station, and extra-vehicular activities are stopped.
This next movie explains in more detail how our sun works.
As you can see from the videos, we are researching our sun/star extensively these days, with US,ESA and Japanese spacecraft focused on it's activity, so we can better understand the effects of the sun on our climate.
This is what a beautiful sunset looks like on Earth:
And this is what it looks like from Mars:
Our Sun is the most well-known (and most visible) example of a G V star (see above). In astronomy, a type G V star is a main-sequence star of spectral type G and luminosity class V. Like other main-sequence stars, a G V star is in the process of converting hydrogen to helium in its core by means of nuclear fusion. Each second, it fuses approximately 600 million tons of hydrogen to helium, converting about 4 million tons of matter to energy. Other G V stars include Alpha Centauri A, Tau Ceti, and 51 Pegasi.
Type G stars range in color from white, for early types like our Sun, to only slightly yellow for the later types. Our own Sun is in fact white. The misconception that it is yellow stems from its appearance through the Earth's atmosphere.
A G V star will fuse hydrogen for, very approximately, 10 billion years, until it is exhausted at the center of the star. When this happens, the star expands to many times its previous size and becomes a red giant, such as Aldebaran (Alpha Tauri). Eventually the red giant sheds its outer layers of gas, which become a planetary nebula, while the core cools and contracts into a small, dense white dwarf.
Our star is but one of hundreds of billions in our galaxy, which for some reason, we call "The Milky Way". Other cultures have called it by many other names.
There are now classifications for tons of different star types according to their size, their age and their composition.
For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star's interior and then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars.
Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. The image below is the star Betelgeuse. It's a very old star but is easy to see in the Orion Constellation. In the picture, Betelgeuse is the star at the top left.
The total mass of a star is the principal determinant in its evolution and eventual fate.
Other characteristics of a star are determined by its evolutionary history, including the diameter, rotation, movement and temperature.
Stars begin as collapsing clouds of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion.
Once the hydrogen fuel at the core is exhausted, those stars having at least 0.4 times the mass of the Sun expand to become red giants,
The star then evolves into a degenerate form (like your author), recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.
IMPACT ON OUR CULTURES
Historically, stars have been important to civilizations throughout the world.
They have been used in religious practices and for celestial navigation and orientation.
Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere, and that they did not move. This inference was the popular thought in the west for over 1500 years (since the Romans).
Ancient astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun. The motion of the Sun against the background stars (and the horizon) was used to create calendars, which could be used for agricultural practices.
The Gregorian calendar, currently used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to the nearest star, our Sun.
The oldest accurately dated star chart appeared in Ancient Egypt in 1,534 BCE.
Islamic astronomers gave to many stars Arabic names which are still used today, and they invented numerous astronomical instruments which could compute the positions of the stars.
In the 11th century, Abū Rayhān al-Bīrūnī described the Milky Way galaxy as multitude of fragments having the properties of nebulous stars, and also gave the latitudes of various stars during a lunar eclipse in 1019.
In this section, the message is how we humans came to rethink the nature of stars. In the late 15th century, we in the western world were just beginning to realize that we were heliocentric -- that the Earth rotated around the sun, and not geocentric -- that everything rotated around Earth. This was a major change in human thought. Correspondingly, the notion that the stars were fixed in the sky was also giving way to the realization that stars moved, and that there were more objects in the sky than stars. Again, this was a major evolution in human thought. This is how this thinking evolved:
Despite thinking that the heavens did not move, Chinese astronomers were aware that new stars could appear. This was especially true when they, along with most of the world saw several supernovae around 1000 AD. They lit up the sky for a period as long as a few weeks to months. The image to the left is of the Crab Nebula that was formed after a star went supernova around 1000AD. When I say that date, please remember this: This particular star was about 5000 light years away. So while we saw the event in 1000AD, the event had actually occurred 5000 years earlier!
Early European astronomers such as Tycho Brahe identified new stars in the night sky (later termed novae - latin for new), suggesting that the heavens were not immutable (fixed).
In 1584 Giordano Bruno suggested that the stars were actually other suns, and may have other planets, possibly even Earth-like, in orbit around them, an idea that had been suggested earlier by such ancient Greek philosophers as Democritus and Epicurus.
By the following century the idea of the stars as distant suns was reaching a consensus among astronomers. To explain why these stars exerted no net gravitational pull on our solar system, Sir Isaac Newton suggested that the stars were equally distributed in every direction, an idea prompted by the theologian Richard Bentley.
The Italian astronomer Geminiano Montanari recorded observing variations in luminosity of the star Algol in 1667. He did not understand he was seeing a binary star -- where the twin stars were rotating around each other.
Edmond Halley (he discovered the comet) published the first measurements of the proper motion of a pair of nearby "fixed" stars, demonstrating that they had changed positions from the time of the ancient Greek astronomers Ptolemy and Hipparchus.
The first direct measurement of the distance to a star (61 Cygni at 11.4 light-years) was made in 1838 by Friedrich Bessel. Parallax measurements demonstrated the vast separation of the stars in the heavens. This again was revolutionary thinking. Humans were rapidly expanding their concepts of Space, Time, Biology. As the Firesign Theatre group said 1. "We're all Bozos on this Bus" and 2. "Everything we know is Wrong". Imagine the impact on the psyche of the scientists and intelligentsia of the times!
William Herschel was the first astronomer to attempt to determine the distribution of stars in the sky. During the 1780s, he performed a series of measurements in 600 directions, and counted the stars observed along each line of sight. From this he deduced that the number of stars steadily increased toward one side of the sky, in the direction of the Milky Way core.
His son John Herschel repeated this study in the southern hemisphere and found a corresponding increase in the same direction. In addition to his other accomplishments, William Herschel is also noted for his discovery that some stars do not merely lie along the same line of sight, but are also physical companions that form binary star systems. (NB. This is one of the first fine art photographs. It was taken in 1867 by Julia Margaret Cameron, one of my favorite photographers).
The science of stellar spectroscopy (identifying the luminosity and composition) was pioneered by Joseph von Fraunhofer and Angelo Secchi. By comparing the spectra of stars such as Sirius to the Sun, they found differences in the strength and number of their absorption lines—the dark lines in a stellar spectra due to the absorption of specific frequencies by the atmosphere. In 1865 Secchi began classifying stars into spectral types. However, the modern version of the stellar classification scheme was developed by Annie J. Cannon during the 1900s.
Observation of double stars gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius, and inferred a hidden companion. (Sirius is the brightest star in the sky and for a time, was the basis for the ancient Egyptian Calendar).
Edward Pickering discovered the first spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104 day period. Detailed observations of many binary star systems were collected by astronomers such as William Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of the orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.
The twentieth century saw increasingly rapid advances in the scientific study of stars. Without getting too heavy (if I'm not already), Important conceptual work on the physical basis of stars occurred during the first decades.
In 1913, the Hertzsprung-Russell diagram was developed, propelling the astrophysical study of stars. Successful models were developed to explain the interiors of stars and stellar evolution. The spectra of stars were also successfully explained through advances in quantum physics. This allowed the chemical composition of the stellar atmosphere to be determined.
SO WHERE CAN WE SEE TODAY?
With the exception of supernovae, even with our most powerful telescopes, we have only been able to image and see individual stars that are primarily in our Local Group of galaxies (when I say local, I do not mean the local pub, I mean within about 5000 light years!). That's not too shabby as these local galaxies hold about 1 trillion stars. Today, our science is sufficient to make fairly detailed star catalogs for our galaxy, which itself, contains between 100 and 300 billion stars.
There are a few exceptions. We have seen some individual stars in the M100 galaxy of the Virgo Cluster, that is about 100 million light years from the Earth! In the Local Supercluster, it is possible to see star clusters (next chapter).
However, outside the Local Supercluster of galaxies, neither individual stars nor clusters of stars have been observed. The only exception is a faint image of a large star cluster containing hundreds of thousands of stars located one billion light years away—ten times the distance of the most distant star cluster previously observed.
Well, that's about all I know about Stars. I hope you enjoyed this. I welcome comments and corrections.