The Planet Neptune

Adams and Leverrier predicted the existence of the planet Neptune back in 1845. Neptune is very difficult to observe from the earth because of the distance, and the earth's atmosphere. The closest and best look that we have had at Neptune was provided by the Voyager 2 flyby in 1989.

Neptune is the most distant planet from the sun. It was once believed that Pluto was a planet Pluto beyond Neptune, but astronomers have declared that Pluto is not a planet but one of many dwarf planets in our solar system.

Neptune has a diameter that is about four times larger than earths. Computer models suggest that Neptune has a rocky core that comprises about 15 earth masses at the center, but there is no confirmation of this theory. Neptune has a rotation period of about 16 hours. Neptune has an internal heat source, and it produces about 2.7 times more heat than it absorbs.

Strong winds, bright, high-altitude clouds, and two large dark spots attributed to long-lived giant storm systems were reveled by the 1989 Voyager flyby. Wind speeds are as high as 739 miles per hour. The larger of the two dark spots that Voyager reveals is called the "Great Dark Spot."

Neptune's rings were first detected from Earth in 1983, but no real study could be conducted at that time. It wasn't until the data supplied by the Voyager 2 flyby was available that any determination about the rings could be disseminated. There are two bright rings and two fainter rings. The rings rotate in the same direction as Neptune, and they are all close to the equator.

Triton and Nereid are the two large moons of Neptune and are visible from Earth. Voyager 2 discovered six more moons, and one of them is actually larger than Nereid; but it is difficult to see because it orbits so close to Neptune.

The Planet Uranus

William Herschel accidentally discovered the planet Uranus in 1781. Uranus had been seen in the sky many times before, but it had always been dismissed as a star.

Uranus is made up mostly of hydrogen and helium like Jupiter and Saturn, but it has higher concentrations of heavy elements. Our closest look at Uranus was given to us by Voyager 2 in 1986.

Uranus is one of the four "Giant Gas Planets" (the other three are Jupiter, Saturn, and Neptune). The rings around Uranus are made up of rock, rather than ice like those of Saturn. We do know that there are five large moons and at least 10 smaller moons. The rotational period of Uranus is a little over 17 hours, and its orbital period is 84 years.

Uranus appears to be a giant blue featureless ball. Voyager 2 got relatively close to Uranus and could not pick up any distinguishable features. It is believed that the upper atmosphere of Uranus has a high-level petrochemical haze that probably obscures features lower in the atmosphere. The blue color is caused by methane gas in the atmosphere. Methane gas absorbs red and orange light, which leaves more blue light.

Voyager 2 confirmed that Uranus has a magnetic field. The field is more than 50 times stronger than that of Earth and is tilted about 60 degrees.

The latest scientific data suggests that Uranus is surrounded by at least five rings. The brightest ring is called the Epsilon ring. Voyager 2 located two small shepherd moons for it, one just inside and one just outside. These two shepherd moons have been named Ophelia and Cordelia.

The most interesting moon that circles Uranus is Miranda. Miranda isn't very big, but it has some very interesting geological features.

The Planet Saturn

The planet Saturn resembles the planet Jupiter in many ways. Saturn is also a planet that has very little if any solid matter. It is believed that there is a hard rocky core, but that belief is unproven (albeit very probable) theory. Saturn, like Jupiter, is made up of gases and liquids. There is an internal heat source. We know this because Saturn radiates more energy than it receives.

Saturn rotates fast. It makes one complete revolution once every 10 or 11 hours. Saturn rotates around the sun once every 29.5 years. Because it rotates so fast, Saturn is flattened at the poles, making it an almost oblate planet.

The space probes, Pioneer 11, Mariner 11 and 12, and Voyager I and II, provide the best and most accurate information to astronomers about the planet Saturn, even though the Hubble space telescope takes very good images of Saturn. The space probes get "up close and personal," so to speak, and have provided information about Saturn that nothing else could have provided.

Wind velocity on Saturn is extreme. Wind speeds of more than 1,118 mph have been recorded. Unlike the winds on Jupiter, however, wind speeds on Saturn do not seem to be closely related to the positions of the belts and bands.

The rings around Saturn are one of the most interesting features. Really high-resolution pictures taken on the Voyager missions tell us that the rings are actually made up of hundreds of thousands of very small rings. The evidence suggests that the rings are composed of particles that are mostly ice crystals. A lot has been learned about the rings of Saturn over the last 20 or so years, but there is so much about them that is still a mystery.

The Planet Jupiter

The planet Jupiter is was named by the Romans after their god Jupiter, who was also sometimes called Jove. Jupiter is the largest planet in our solar system by far. It is more than twice as large as all of the other planets combined.

Jupiter might well have become a star when it was born, had it been larger. There is very little (if any) solid matter on Jupiter. If there is any at all, it is hidden deeply inside the planet. Jupiter is made up entirely (as far as we know now) of gases and liquids.

The very composition of Jupiter means that its parts do not rotate at the same speed, but rotation is fast. Jupiter makes one complete revolution in a little less than 10 hours. This very fast rotation plus the makeup of gases and liquids are what causes the bulge at the equator of Jupiter.

Jupiter has an internal heat source. We are sure of this because it actually emits more radiation than it gets from the sun.

There are four large moons and dozens of small moons that rotate around Jupiter, making it a kind of small solar system unto itself.

One of the more outstanding features about Jupiter is the never-ending hurricane called "The Giant Red Spot" in the southern hemisphere. This disturbance has been going on for the last 400 years that we know of. It never abates…probably because it never passes over land (there isn't any) like hurricanes on earth do.

An explanation for the color of the clouds on Jupiter still eludes astronomers. With the conditions that exist, clouds should be colorless, but they are anything but colorless, and they change color over time. We still have a lot to learn!

The Planet Mars

Earthlings have always been fascinated by the planet, Mars. The "little green men from Mars" invading Earth has been the stuff of science fiction for decades. On October 30, 1938, Orson Welles did a dramatization of H. G. Wells's "War of the Worlds" on the Mercury Theatre on the Air radio show. During the four commercial breaks of the program, there were disclaimers aired telling the audience that the content was fictional. Still, panic erupted in towns across America.

There was a new 40-inch telescope being built by the University of Chicago in 1895. An astronomy professor, Samual Leland Phelps, wrote a book about the project called "World Making." In the book, the professor wrote, "It will be possible to see cities on Mars, to detect navies in [its] harbors, and the smoke of great manufacturing cities and towns...Is Mars inhabited? There can be little doubt of it...conditions are all favorable for life, and life, too, of a high order. Is it possible to know this of a certainty? Certainly." Well…not exactly, professor!

From observations of Mars from stationary observatories on earth, astronomers concluded that:

1. The reddish color of Mars is caused by red rocks and dust.
2. The polar ice caps increase and decrease according to seasons.
3. There are what were thought to be canals on the surface. (That has since been disproved)
4. There are areas of Mars that change color. (It was believed that this was vegetation, but that has been disproved, as well.)
5. Mars has an atmosphere.

Space probes, beginning with Mariner 4, 6, 7, and 9 from 1965 through 1971, and the Viking 1 and 2 probes in 1976, disproved many of the previously held beliefs about Mars. We are learning more about Mars all the time, and one important fact is that there aren't any little green (or any other color) men living on Mars.

The Planet Venus

Back in 1686, a French scholar by the name of Bernard de Fontenelle, wrote, "I can tell from here...what the inhabitants of Venus are like; they resemble the Moors of Granada; a small black people, burned by the sun, full of wit and fire, always in love, writing verse, fond of music, arranging festivals, dances, and tournaments every day."

Nice try, Bernard, but you had it all wrong. Back in those days, it was generally accepted that Venus was much like Earth. Venus is about the size of Earth, but that's pretty much where all resemblance ends.

Almost all of the planets in our solar system travel around the sun in a counterclockwise direction…all of them except Venus and Uranus, that is. Venus (as well as Uranus) travels clockwise. All of the planets except Venus and Uranus rotate on their axis in a counterclockwise direction. Venus and Uranus rotate clockwise.

Venus also rotates really slowly, too. A "day" on Venus would equal about 243 Earth days. Venus is covered by a thick layer of clouds that make studying the surface very difficult. In the last 30 years, however, astronomers have learned how to "see" through the thick cloud cover.

In 1962, Mariner 2 was the first spacecraft to go by Venus. Since then, there have been 20 more missions. The first hard landing on Venus was done by Venera 4 in 1967. The first soft landing was done by Venera 7 in 1970.

In 1989, the Magellan spacecraft probe was launched. Magellan rotated around Venus from 1990 to 1995. It then burned up in the atmosphere of Venus. Magellan bounced radar signals off the surface of Venus and transmitted the data back to Earth.

The Planet Mercury

The planet Mercury is the smallest planet in our solar system, and the one that is closest to the sun. This proximity to the sun has made it difficult to study the planet. The closest look that we have ever had was in 1974 and 1975 when the unmanned Mariner 10 was sent to map the surface of Mercury. Only about 40% to 45% of the surface was actually mapped.

Mercury resembles our moon in appearance as it is heavily cratered. There is an atmosphere of sorts on Mercury but not one that would sustain life as we know it. The atmosphere is very unstable and is made up of hydrogen, helium, oxygen, sodium, calcium and potassium.

Records of studies made of Mercury date back to 300 BC. The name "Mercury" was given to the planet by the Romans after the god Mercurius. Chinese, Korean, Japanese, and Vietnamese cultures refer to Mercury as the water star based on the Five Elements. Other cultures have called the planet by other names. The Babylonians, for example, called the planet Nabu or Nebu after the messenger to the Gods in their mythology.

The temperature on Mercury varies. The average temperature is 179° C, but the extremes are a low of -183 °C to a high of 427° C. Sunlight on Mercury is six and a half times stronger than it is on Earth.

There is evidence that water does exist on Mercury despite the extreme temperature variations. The bottoms of some of the deepest craters near the poles are never exposed to direct sunlight. Temperatures in these areas remain far lower than the global average, so it is possible for ice to exist.

Types of Binary Stars

Looking through a telescope at the sky there is very little information we can gain from them. To be sure, we know what color they are and we can see that some are more luminous than others. If we use a spectrograph we can tell what elements they are made up from. From these facts alone, it is difficult to tell just how much mass they contain.

By looking at pairs of stars that orbit one another we can try to answer the question, how much mass do the stars have?

Binary stars can be of two fundamental types:

• Visual Binaries
• Optical Doubles

Visual Binary (Alberio Binary)

Visual Binaries are stars that are clearly gravitational associated with one another. They orbit each other around a common center called the barycenter. Visual binaries can be seen optically through a telescope. Only a small portion of binary stars are visual binaries. In order to see a visual binary, the stars must be separated by fairly wide distances, and the orbital periods are usually very long.

Optical Doubles are stars that appear to lie close together, but in fact do not, they only appear to us from our earthly observation to be close together. One of the stars in the pair is actually behind the first star and very far away. The stars of an optical double are not gravitationally bound.

William Herschel began looking for optical doubles in 1782 with the hope that he would find a measurable parallax, by comparing a close star to the more distant star in an optical double.

Herschel did not find any optical binaries, but he did catalog hundreds of visual binaries. In 1804 Herschel had so many measurements of visual binaries that he concluded that a pair of stars known as Castor were orbiting one another. This was an important discovery, because it was the first time observational evidence clearly showed two objects in orbit around each other outside of the influence of our own Sun and Solar System.

Spectroscopic Binary

It is also possible to detect binary stars using a spectroscope. If two stars are orbiting each other they will both produce a spectrum. If the stars are close to being the same brightness it is possible to see different spectral lines from both stars. These stars are of particular interest because it can be used to determine the radial velocity of the orbit of the two stars. Stars appear red shifted when receding away from the earth and blue shifted as they approach. This effect is caused by the Doppler effect which distorts arriving light waves from the stars depending on the direction if their motion. A Spectroscopic binary will alternate between blue and red shifted spectral lines.

Spectroscopic binaries are not detectable if we are seeing the star head on because no Doppler shifts would be present in the spectrum. If the Doppler shifts are present in a single line of the spectrum, we are seeing the light from only one star and we call this a single-line spectroscopic binary. If we can see the light from both stars the Doppler shifts will alternate, split and merge depending on the positions of the two stars in their orbits. This is called a double-line spectroscopic binary.

One very important detail, we do not know how the orbits of the two stars are inclined to earth. This inclination could be any angle, for that bit of information we have to go back to visual methods in order to see the individual stars to determine the inclination of their orbits relative to earth. Even so we can not for certain determine the true inclination of the orbit so our mass calculation is only a lower limit to the masses of the two stars.

Radial velocities permit astronomers to compute the total mass for the two stars, they do not provide the masses for the individual stars and other methods must be used to make that determination..

Eclipsing Binary

Another type of binary called the Eclipsing binary can be studied. The information gathered can be used to calculate the individual stellar masses and the diameters of the individual stars. It is rare to find two stars in orbit around one another to have orbital inclination where the stars pass in front of one another to form one point of light as seen from earth.

When the orbital inclination if the eclipsing binary is edge on to earth, the stars will seem to pass in front of one another as they orbit, when the light from the brighter star is eclipsed we will see a deep decline in the amount of light received from the star (6/25/95 in Figure 1) we call this primary minimum, also when the light from the dimmer star is blocked by the brighter the light received declines again, but not so deep and we call this secondary minimum (see 6/9/95 in Figure 1) , otherwise we are able to collect some or all of the light from both stars.

The pattern of these light changes is called a light curve and the data for it gathered by the use of a photometer, making periodic measurements until the eclipsing binaries produce a complete orbital cycle.

We use the mass vs. luminosity relationship to determine what the difference is between the individual masses, then using the mass of the entire system calculated from the radial velocity information, we can determine what the individual masses of the two stars should be. The photometeric data removes some of the uncertainty in regard to the inclination because the shapes of the light curves will be different for a partial eclipse than for a total eclipse.

ALGOL is one of the best known and most studied eclipsing binary stars. ALGOL is normally about 2.3 magnitude, but every 10 hours or so it will dim to about 3.4 magnitude, in other words ALGOL becomes 68% dimmer. I suspect that humanity has known about ALGOL’s behavior for quite some time, since the Arabic name of ALGOL means "Demons Head", and ALGOL is associated with the severed head of Medusa. ALGOL is often referred to as the winking eye of the demon.

An eclipsing binary occurs when the orbital plane of the binary system is exactly When one star passes directly in front of the other, as viewed from Earth, we seen an eclipsing binary perpendicular to the plane of the sky.

Learn Astronomy Basic (part 2)

Learn Astronomy Basic part 2. After we talk about parallax, now we will discuss about angular diameter.

Definition

The angle that the actual diameter of an object makes in the sky; also known as angular size or apparent diameter. The angular diameter of an object as seen from a given position is the “visual diameter” of the object measured as an angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its center that is perpendicular to the viewing direction. Because of foreshortening, it may be quite different from the actual physical diameter for an object that is seen under an angle. For a disk-shaped object at a large distance, the visual and actual diameters are the same.The Moon, with an actual diameter of 3,476 kilometers, has an angular diameter of 29′ 21″ to 33′ 30″, depending on its distance from Earth. If both angular diameter and distance are known, linear diameter can be easily calculated.

The Sun and the Moon have angular diameters of about half a degree, as would a 10-centimeter (4-inch) diameter orange at a distance of 11.6 meters (38 feet). People with keen eyesight can distinguish objects that are about an arc minute in diameter, equivalent to distinguishing between two objects the size of a penny at a distance of 70 meters (226 feet). Modern telescopes allow astronomers to routinely distinguish objects one arc second in diameter, and less. The Hubble Space Telescope, for example, can distinguish objects as small as 0.1 arc seconds. For comparison, 1 arc second is the apparent size of a penny seen at a distance of 4 kilometers (2.5 miles).

The angular diameter is proportional to the actual diameter divided by its distance. If any two of these quantities are known, the third can be determined.

For example if an object is observed to have an apparent diameter of 1 arc second and is known to be at a distance of 5,000 light years, it can be determined that the actual diameter is 0.02 light years.

Formulas

 

The angular diameter of an object can be calculated using the formula:

in which δ is the angular diameter, and d and D are the visual diameter of and the distance to the object, expressed in the same units. When D is much larger than d, δ may be approximated by the formula δ = d / D, in which the result is in radians.

For a spherical object whose actual diameter equals d_act, the angular diameter can be found with the formula:

For practical use, the distinction between d and d_act only makes a difference for spherical objects that are relatively close.

Estimating Angular Diameter

This illustration shows how you can use your hand to make rough estimates of angular sizes. At arm’s length, your little finger is about 1 degree across, your fist is about 10 degrees across, etc.

Use in Astronomy

In astronomy the sizes of objects in the sky are often given in terms of their angular diameter as seen from Earth, rather than their actual sizes.

The angular diameter of Earth’s orbit around the Sun, from a distance of one parsec, is 2″ (two arcseconds).

The angular diameter of the Sun, from a distance of one light-year, is 0.03″, and that of the Earth 0.0003″. The angular diameter 0.03″ of the Sun given above is approximately the same as that of a person at a distance of the diameter of the Earth.

This stats shows the angular sizes of noteworthy celestial bodies as seen from the Earth:

Sun	31.6′ – 32.7′
Moon	29.3′ – 34.1′
Venus	10″ – 66″
Jupiter	30″ – 49″
Saturn	15″ – 20″
Mars	4″ – 25″
Mercury	5″ – 13″
Uranus	3″ – 4″
Neptune	2″
Ceres	0.8″
Pluto	0.1″

* Betelgeuse: 0.049″ – 0.060″
* Alpha Centauri A: ca. 0.007″
* Sirius: ca. 0.007″

This meaning the angular diameter of the Sun is ca. 250,000 that of Sirius (it has twice the diameter and the distance is 500,000 times as much; the Sun is 10,000,000,000 times as bright, corresponding to an angular diameter ratio of 100,000, so Sirius is roughly 6 times as bright per unit solid angle). The angular diameter of the Sun is also ca. 250,000 that of Alpha Centauri A (it has the same diameter and the distance is 250,000 times as much; the Sun is 40,000,000,000 times as bright, corresponding to an angular diameter ratio of 200,000, so Alpha Centauri A is a little brighter per unit solid angle).

The angular diameter of the Sun is about the same as that of the Moon (the diameter is 400 times as large and the distance also; the Sun is 200,000-500,000 times as bright as the full Moon (figures vary), corresponding to an angular diameter ratio of 450-700, so a celestial body with a diameter of 2.5-4″ and the same brightness per unit solid angle would have the same brightness as the full Moon).

Even though Pluto is physically larger than Ceres, when viewed from Earth, e.g. through the Hubble Space Telescope, Ceres has a much larger apparent size.

While angular sizes measured in degrees are useful for larger patches of sky (in the constellation of Orion, for example, the three stars of the belt cover about 3 degrees of angular size), we need much finer units when talking about the angular size of galaxies, nebulae or other objects of the night sky.

Degrees, therefore, are subdivided as follows:

* 360 degrees (º) in a full circle
* 60 arc-minutes (′) in one degree
* 60 arc-seconds (′′) in one arc-minute

To put this in perspective, the full moon viewed from earth is about ½ degree, or 30 arc minutes (or 1800 arc-seconds). The moon’s motion across the sky can be measured in angular size: approximately 15 degrees every hour, or 15 arc-seconds per second. A one-mile-long line painted on the face of the moon would appear to us to be about one arc-second in length.