Facts about the Sun
(from Wikipedia)
Mean distance
from Earth
1.496*10^11 m
Travel time at light speed
8.31 min
Visual brightness (V)
−26.74m
Absolute magnitude
4.83m
Luminosity
3.8x10^26 Watts
Spectral classification
G2V
Metallicity
Z = 0.0177
Angular size
31.6′ - 32.7′
Age
4.6x10^9 years
Orbital characteristics
Mean distance
from Milky Way core
~2.5*10^20 m
26 000 light-years
Galactic period
(2.25–2.50)*10^8 a
Velocity
~2.20*10^5 m/s
(orbit around the center of the Galaxy)
~2*10^4 m/s
(relative to average velocity of other stars in stellar neighborhood)
Physical characteristics
Mean diameter
1.392*10^9 m
Equatorial radius
6.955*10^8 m
Equatorial circumference
4.379x10^9 m
Flattening
9*10^−6
Surface area
6.087*10^18 m^2 or 11,990 Earths
Volume
1.41*10^27 m^3 or 1,300,000 Earths
Mass
1.9891*10^30 kg or 332,946 Earths
Different Densities
Average density
1.408 *10^3 kg/m^3
Core: 1.5*10^5 kg/m^3
lower Photosphere:
2*10^-4 kg/m^3
lower Cromosphere:
5_10-6 kg/m^3
Avg. Corona:
10_10-12kg/m^3
Gravitational Quantities
Equatorial
274.0 m/s^2
surface gravity
27.94 g, 28 * Earth surface gravity
Escape velocity (from the surface)
617.7 km/s
55*Earth's escape velocity
Temperature of surface (effective)
5 778 K
Temperature of corona
~5*10^6 K
Temperature of core
~15.7*10^6 K
Luminosity (Lsol)
3.846*10^26 W
~3.75*10^28 lm
~98 lm/W efficacy
Mean Intensity (Isol)
2.009*10^7 W m^-2 sr^-1
Rotational characteristics
Obliquity
(to the ecliptic)
7.25 degrees
(to the galactic plane)
67.23 degrees
Right ascension of North pole
286.13 degrees
19 h 4 min 30 S
Declination of North pole
+63.87 degrees
63 deg 52 min North
Sidereal Rotation period
(at 16_ latitude)
25.38 days
25 d 9 h 7 min 13 s
(at equator)
25.05 days
(at poles)
34.3 days
Rotation velocity
(at equator)
7.284*10^3 km/h
Photospheric composition (by mass)
Hydrogen 73.46 %
Helium 24.85 %
Oxygen 0.77 %
Carbon 0.29 %
Iron 0.16 %
Sulfur 0.12 %
Neon 0.12 %
Nitrogen 0.09 %
Silicon 0.07 %
Magnesium 0.05 %
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The Sun
An Introduction
Written by Katie Whitman with information from NASA.gov, Wikipedia, HowStuffWorks.com, Geology.com, Space.com
According to astronomers' best theories, about 4.6 billion years ago, a dust cloud collapsed and most of the mass gathered at the center of the cloud. The mass was pulled together so strongly that enough pressure was created in the center of this collapsed cloud to ignite nuclear fusion between Hydrogen atoms. Upon ignition, that collapsed dust cloud became our young Sun. Much of the remaining material from the dust cloud fell into orbits around the Sun, colliding and coalescing, eventually forming the planets, moons, comets, and asteroids that we see today.
Because of the ongoing nuclear fusion within the Sun's core, the Sun remains very hot. At the Sun's visible surface, called the photosphere, the temperature is about 5800 Kelvin (or 6073 degrees Celcius). The Sun has an atmosphere just above the photosphere called the chromosphere. The chromosphere is a thick layer of Helium and Hydrogen with temperatures of about 4000K near the photosphere, but increasing rapidly to temperatures of tens of thousands of degrees K near the top of the chromosphere as it transitions into the corona. The corona extends several million miles into space and can reach temperatures of 2 million degrees Kelvin. Despite being so hot, the corona is not visible under normal conditions because it is very thin and does not emit enough light to compete with the brightness of the daylight sky. It is possible to observe the corona during eclipses, however, when the light of the surface of the sun is blocked and the sky darkens.
The temperature in the corona is so hot that nearly all of the atoms in the corona are ionized, i.e. electrons have been stripped from the atoms, creating a soup of positively and negatively charged particles. Also due to these extreme temperatures, these particles are moving extremely fast, fast enough to escape the gravity of the sun. This continual outflow of escaping particles creates what is known as the solar wind. This wind is not constant over the surface of the sun, rather it varies in speeds from 300 km/s to 800 km/s. The higher speed winds are observed over coronal holes while lower speed winds are seen over streamers.
The charged particles of the solar wind constantly interact with the Earth's magnetic field, which acts as a shield against the potentially damaging particles. We are able to view some of these interactions when the charged particles of the solar wind stream down the Earth's magnetic field lines and collide with atoms in our own atmosphere, exciting those atoms. These excited atmospheric atoms emit light as they return to their normal states, creating a beautiful glow that we observe as the aurora borealis. Data from the NASA satellite TRACE has indicated that there are magnetic connections, described as "ropes," between the Sun and the Earth along which charged particles stream. It has long been known that aurora borealis are more active during the spring and autumn and this new data indicates that during the equinoxes the Earth's magnetic poles are best oriented for these "ropes" to link up.
While the solar wind can create beauty like the Northern Lights, these charged particles can also do a lot of damage. Electronics in satellites orbiting the Earth could be damaged by a strong burst of particles, astronauts are in danger of receiving high doses of radiation, and particles that do make it through the Earth's atmosphere have been known to fry electronics and cause transformers to burst into flame, causing blackouts. Even airlines reroute their flights away from the polar regions where radiation is highest when a particularly large solar storm occurs.
The Sun is also active in many ways other than the solar wind. The Center for Computational Heliophsyics in Hawaii (C2H2) studies many of the behaviors of the active sun. For example, some of the most dramatic events that are observed on the Sun are coronal mass ejections (CMEs). CMEs are huge bubbles of gas that are ejected from the surface of the Sun all at once (as supposed to in a stream like the solar wind). When a CME hits Earth, it compresses the Earth's magnetic field, causing a tail on the side opposite of impact. When the tail of the magnetic field reconnects, it creates trillions of watts of power that are directed back towards the Earth and can cause the catastrophic events described in the paragraph above. CMEs can move slowly or quickly and occasionally one might overtake a previously ejected CME, causing what we call at C2H2, CME Cannibalism.
CMEs may be associated with solar flares, huge explosions that occur on the surface of the Sun, releasing the equivalent of as much as a billion megatons of TNT in the form of gamma rays and X-rays, protons and electrons, or mass flows (like CMEs). Solar flares happen in a matter of minutes or tens of minutes and are very often generated in sunspots, regions of intense magnetic activity.
The sun's magnetic field, another topic of study at C2H2, is very interesting and complicated. At the simplest level, the sun's magnetic field looks like a bipole, same kind of magnetic field that a bar magnet produces (see image above). However, because the Sun is not a solid body it undergoes differential rotation. In other words, different parts of the Sun rotate at different speeds. This results in the magnetic fields lines getting all twisted up and forming very complicated structures. Sometimes the magnetic field will puncture the surface of the sun and it is at this location where sunspots form. As you might suspect, one half of the sunspot has a magnetic field that is exiting the surface of the sun while the other half has a magnetic field that reenters the surface. Sunspots are so dark because they are about 1800 K cooler than the surrounding photosphere, appearing less bright in comparison.
The Sun undergoes an approximately 11 year cycle in which there are many sunspots, referred to as solar maximum, to very few sunspots, referred to as solar minimum, and back again. The Sun's magnetic field as a whole also follows this 11 year cycle, flipping polarity at about the time of solar maximum and reaching peak magnetic field strength at the time of solar minimum. Curiously, an extended period between 1645 and 1715 during which sunspots were very rare, known as the Maunder Minimum, corresponded with the coldest part of the Little Ice Age, a time during which Europe and North America experienced abnormally cold winters. Scientists have yet to determine whether sunspot activity and temperature on Earth has a truly causal connection.
The Sun is a much more complex body than most of us may have ever expected and perhaps even more so than astronomers suspect today. It is the centerpiece of the solar system, the energy that gives us life, a complex system that continually buffets the Earth with its solar winds, and surprises us with its extraordinary activity. Astronomers study all aspects of the Sun from both Earth and space and we strive to continually further our understanding of this phenomenal heavenly body through our work at the Center for Computational Heliophsyics in Hawaii.