Archive for March, 2009

Question of the Week: What does the color of a star tell us?

Reader Questions | Posted by Drew on Behalf of Dr. Bob.
Mar 29 2009

morgan-keenan_spectral_classification

For the purposes of this article, the color or spectral type of the star tells us the star’s temperature: O, B, A, F, G, K, M.  The hottest stars are the “O” type blue stars.  An example of a blue star is Delta Orionis (Mintaka).  It is the third star of Orion’s belt, going up from left to right.  Its temperature is from 28,000 to 50,000 degrees Kelvin.  The next spectral type is the “B” type Blue star.  It has a temperature range from 10,000 to 28,000 degrees Kelvin.  Rigel is an example of this type of star.  The third spectral class of star is the “A” type Blue-White star.   These stars range in temperature from 10,000 to 7,500 degrees Kelvin.  Sirius, the brightest star in the night sky, is a type “A” star.

The type “F” star is the fourth class of star and is white.  Its temperature range is from 7,500 to 6,000 degrees Kelvin.  Procyon, Little Dog, is a good example of a “F” type star.  The fifth class of star is the “G” type star.  It is yellow and the Sun is a classic example of one of these.  These stars have a temperature from 6,000 to 5,000 degrees Kelvin.  The sixth type star is a “K” star or an orange star.  These cooler stars have a temperature range from 5,000 to 3,500 degrees Kelvin.  Arcturus, in Bootes, is a good example of one of these stars.  Finally, the coolest star type is the “M” type star is a orange-red star.  These have a temperature range from 3,500 to 2,500 degrees Kelvin.  Antares in Scorpio is a good example of one of these stars.

Add star color to your observing list sometime.  See if you can identify at least one of each spectral class of stars.  Remember, if you “defocus” your eyepiece as you are looking at the star, you will be able to see its color more vibrantly.

hrdiagram

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Celestial Feature of the Week: The Sickle

Celestial Feature of the Week | Posted by Drew on Behalf of Dr. Bob.
Mar 29 2009
Leo by Dibuix

Leo by Dibuix

There are a number of asterisms in the night sky but few are more easily seen than the “backward question mark” or “sickle” of Leo, the Lion.  This asterism represents the head and shoulders of the lion.  It is composed of six prominent stars with Regulus, at the base, being the brightest.  Regulus is a “B” type Blue star, with a temperature a little more than 10,000 degrees Kelvin.  It lies about 78 light-years from Earth and has a magnitude of 1.4.

Regulus

Regulus

The next star in the sequence is Eta Leonis.  This star is about 2,000 light-years from Earth and is a Blue-Super Giant.  It has a magnitude of 3.5.  The next star is Algieba (Gamma Leonis).  Algieba is a double star that can be split with a small telescope.  This binary pair is about 126 light-years from Earth and is made up of a giant K star that has a surface temperature of 4,470 Kelvin and is about 23 times larger than the Sun. The companion star has an apparent magnitude of +3.51 and is a giant G star that has a temperature of 4,980 K.  It is thought to be about 10 times the size of the Sun.

The fourth star is Zeta Leonis.  It is about 260 light-years from Earth and has a magnitude of 3.4.  The fifth star is Mu Leonis.  It is 133 light-years from Earth and shines in at magnitude 3.9.  The last star in the sickle is Epsilon Leonis.   It lies about 250 light-years from Earth and is a magnitude 3.0 star.

Take some time to go out this week and locate the stars of the Sickle and identify them individually.  Oh, by the way
  Saturn is just to the east of Regulus along the Ecliptic, below the hindquarters of the Lion.

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Today in History: 3/29/09 – 4/4/09

Space History | Posted by Drew on Behalf of Dr. Bob.
Mar 29 2009

March 29, 1807: Willhelm Olbers discovered this second asteroid, Vesta, five years and one day after his first discovery.  Vesta is the fourth asteroid to be discovered.

A Fragment of Vesta

A Fragment of Vesta

March 29, 1974: Mariner 10 makes the first flyby of Mercury and returns close-up pictures of the inner most planet of our solar system.

Shot from Mariner 10

Shot from Mariner 10

March 31, 1997: Pioneer 10 mission officially ends. Following its encounter with Jupiter in 1973, Pioneer 10 explored the outer regions of the solar system, studying energetic particles from the Sun (solar wind), and cosmic rays entering our portion of the Milky Way. The spacecraft continued to make valuable scientific investigations in the outer regions of the solar system until its science mission ended March 31, 1997.  Currently Pioneer 10 is more than 98 astronomical units (AUs) from Earth or 13.6 light-hours away.

Pioneer 10's path.

Pioneer 10's path.

April 1, 1997: Comet Hale-Bopp made it closest approach to the Sun.  It was .914 AUs or 84.8 million miles from the Sun.

Hale-bopp

Hale-bopp

April 2, 1845: The first the first daguerreotype (photograph) of the Sun was taken by French physicists Louis Fizeau (1819-1896) and Lion Foucault (1819-1868).

Louis Fizeau

Louis Fizeau

April 3, 1966: Luna 10 becomes the first spacecraft to orbit the Moon.

Luna-10

Luna-10

April 4, 1983: The space shuttle Challenger roared into orbit on its maiden voyage.

April 4, 2000: First commercial mission to Mir.  The cost of keeping a space station in orbit is very expensive.  The Russians tried to commercialize the Mir Space station to finance it.  In a news release, the Russians stated that “Cosmonauts Sergei Zaletin and Alexander Kalery opened the hatch to Mir this morning after successfully docking their Soyuz transport spacecraft to the massive orbital station. They became the first crewmembers to board Mir since 1999, and the cosmonauts will reactivate the station for commercial operations.”  It is ironic that exactly five years later, Mir disintegrated as it reentered the Earth’s atmosphere.

Mir and Atlantis

Mir and Atlantis

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Telescope Corner: What good is a finder scope?

Telescope Corner | Posted by Drew on Behalf of Dr. Bob.
Mar 29 2009

One of the challenges and frustrations for observers is locating a faint object in the telescope.  The typical telescope field of view is less than one degree.  When looking for a specific object through a telescope, one needs to be able to use reference stars to locate the object.  This is easy if there are many prominent reference stars to guide you.  When you are looking in an area where there are few landmarks (skymarks), then the hunting is very difficult.

A finder scope is a small telescope or spotter, which you attach to and align with the telescope.  It has a relative wide field of view and provides the observer with a much easier way to narrow down the area where the object is located.  Once the object is approximately in the field of view of the telescope, the observer can pinpoint it through the telescope.

There are several types of finder scopes and I will discuss them in the next article.

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Weekly Stargazers’ Rating: 3/29/09 – 4/4/09

Stargazer Rating | Posted by Drew on Behalf of Dr. Bob.
Mar 29 2009

The Moon will be waxing from a shallow crescent at the first of the week to Gibbous by the end of the week.  Because it will set early in the evening at the first of the week, the Stargaze Rating for the first couple days in the week are good.  By mid week the moon will be very prominent in the sky, making it difficult to do much deep sky observing except in the early morning.

The Moongaze Rating is very good for the early evening and night as the terminator crosses from the eastern limb to the mid-western meridian.

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Planet Watch: 3/29/09 – 4/4/09

Planet Watch | Posted by Drew on Behalf of Dr. Bob.
Mar 29 2009

For a great website to locate the planets, go to www.nightskyinfo.com.  This site will tell you about the planets and give you a good star map to locate them.

Monday (30th), Mercury is in superior conjunction at 11 p.m. EDT.  This is when Mercury is in line with the Sun and Earth but on the opposite side of the Sun.

Sunset Sky: (Sunset until 11 p.m.)

Venus and Mercury are just in inferior and superior conjunction, respectively.  This means that Venus is between the Sun and Earth while Mercury is on the opposite side of the Sun from Earth.  The result is the same; we can’t see either one for a while because of the Sun’s bright glare.

Late Evening (11 p.m. until 4 a.m.):

Saturn is the only visible planet at this time of day.  Look high in the south to spot Saturn.  It will be very bright and look like a “mislaid” star in Leo, the lion.  If you are not familiar with Leo, take a minute to look at a star map of Leo and then at the sky.   Saturn, by its position against the familiar stars of Leo, will jump right out at you; you won’t be able to miss it.

Predawn Sky:  (4 a.m. until sunrise)

Still not a lot to see in the morning but there is a traffic jam low on the eastern horizon.  Jupiter, Neptune, Mars, Venus, and Uranus are all beginning to rise in the east at sunrise.  Within a week or so, we will be getting a nice view of a couple of these.  Right now, you might be able to see Jupiter if you have a good low horizon.

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Moon Watch: 3/29/09 – 4/4/09

Moon Watch | Posted by Drew on Behalf of Dr. Bob.
Mar 29 2009

Wednesday (1st), the Moon will be in perigee, its closest distance to the Earth, (229,916 miles) at 10:17 a.m. EST

Thursday (2nd), the Moon is in First Quarter at 10:34 a.m. EDT.

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Satellite Watch: 3/29/09 – 4/4/09

Satellite Watch: Satellites of Magnitude 3.0 and brighter | Posted by Drew on Behalf of Dr. Bob.
Mar 29 2009

Sunday, March 29, 2009:

Tropical Rainforest Monitoring Mission (TRMM): 06:43 Hrs, 59Âș altitude in the SSW, rising from the W, heading to the ESE, magnitude 1.6.
Lacrosse 3 Satellite: 21:19 Hrs, 87Âș altitude in the NNE, rising from the NW, heading to the SE, magnitude 2.5.

Monday, March 30, 2009:

Lacrosse 2 Satellite: 05:40 Hrs, 61Âș altitude in the ESE, rising from the S, heading to the NNE, magnitude 2.9.
Tropical Rainforest Monitoring Mission (TRMM): 05:48 Hrs, 50Âș altitude in the ESE, rising from the ESE, heading to the ESE, magnitude 2.1.
Lacrosse 4 Satellite: 06:18 Hrs, 59Âș altitude in the WSW, rising from the NNW, heading to the S, magnitude 2.6.
Cosmos 1143 Rocket Body: 20:43 Hrs., 73Âș altitude in the WSW, rising from the NNW, heading to the S, magnitude 2.9.

Tuesday, March 31, 2009:

Lacrosse 2 Satellite: 06:04 Hrs, 50Âș altitude in the WNW, rising from the WSW, heading to the NNE, magnitude 2.8.
Genesis-1 Satellite: 06:30 Hrs, 81Âș altitude in the ESE, rising from the SSW, heading to the NNE, magnitude 2.8.
Tropical Rainforest Monitoring Mission (TRMM): 06:29 Hrs, 32Âș altitude in the SSW, rising from the WSW, heading to the SE, magnitude 2.3.
Okean O Rocket (Tumbling Rocket Body): 07:02 Hrs., 80Âș altitude in the WNW, rising from the N, heading to the SSW, magnitude 2.8.
Lacrosse 3 Satellite: 20:36 Hrs, 78Âș altitude in the NE, rising from the NW, heading to the SE, magnitude 2.6.
International Space Station (ISS): 21:17 Hrs, 42Âș altitude in the NE, rising from the NW, heading to the NE, magnitude -1.8.

Wednesday, April 1, 2009:

Lacrosse 4 Satellite: 05:51 Hrs, 58Âș altitude in the WSW, rising from the NNW, heading to the S, magnitude 2.6.
Genesis-1 Satellite: 06:28 Hrs, 71Âș altitude in the WNW, rising from the SW, heading to the NNE, magnitude 2.7.
International Space Station (ISS): 21:43 Hrs, 35Âș altitude in the WSW, rising from the WNW, heading to the WSW, magnitude -0.9.

Thursday April 2, 2009:

Lacrosse 2 Satellite: 05:14 Hrs, 60Âș altitude in the NNE, rising from the NNE, heading to the NNE, magnitude 2.9.
International Space Station (ISS): 20:35 Hrs, 51Âș altitude in the NE, rising from the NW, heading to the ESE, magnitude -2.0.
Upper Atmosphere Research Satellite (UARS): 21:29 Hrs, 80Âș altitude in the SW, rising from the NW, heading to the SSE, magnitude 2.2.

Friday, April 3, 2009:

Upper Atmosphere Research Satellite (UARS): 20:37 Hrs, 39Âș altitude in the NE, rising from the NNW, heading to the ESE, magnitude 2.9.
International Space Station (ISS): 21:02 Hrs, 30Âș altitude in the SW, rising from the WNW, heading to the ESE, magnitude -0.7.

Saturday, April 4, 2009:

Okean O Rocket (Tumbling Rocket Body): 06:59 Hrs., 85Âș altitude in the WNW, rising from the N, heading to the SSW, magnitude 2.8.

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Reader Question: Why do stars twinkle?

Reader Questions | Posted by Drew on Behalf of Dr. Bob.
Mar 22 2009

This is a good question and the answer it not immediately obvious.  It is true that stars do twinkle.  We have to consider, what is it that makes the stars twinkle, in the first place.  I want to give you an example that will help you see what is happening.  What do you see when you are looking down a long asphalt road on a hot summer day?  What you see is, the heat waves in the air rising above the road.  It makes the background wavy.

This is the same process that causes stars to twinkle.   The heat from the earth rising in the atmosphere creates currents in the air that makes the light from the star wave slightly.  This waving action makes the star appear to twinkle.  Other than temperature variations, another cause for stars to twinkle is wind or moving masses of air.  When the air moves, the air changes concentration and density.  When this happens, it is like a window glass with waves in it.  The viewing becomes unsteady and star twinkling occurs.

To stargazers, the property of the atmosphere that pertains to twinkling is called “seeing.”   The general rule of thumb is, the steadier the atmosphere; the better the seeing.   When the seeing is good, the object viewed through a telescope is crisp and clean.  When the seeing is poor, the image is blurry.  It is true that the seeing is improved by going to higher elevations.  That is why most serious observatories are located on the top of high mountains.  That is also why the Hubble Space Telescope is so effective; it doesn’t have any atmosphere to have to look through so the seeing is perfect.

There is another dimension of factor of viewing other than seeing; it is called transparency.  This is factor of the atmosphere relates to its clarity.  It is interesting to note that transparency and seeing do not necessarily go together.  It is not uncommon for a hazy night with fair to poor transparence to have great seeing.   It is also just common to have a crystal clear night with great transparency but the atmosphere is terribly unstable and the stars twinkle with poor seeing.

So when do stargazers go outside to observe; when the sky has good transparency or good seeing?   Well, after all is said and done
.  the best time to go out and stargaze is 
  whenever you can!  

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Celestial Feature of the Week: Hydra (M48, M68, and M83)

Celestial Feature of the Week | Posted by Drew on Behalf of Dr. Bob.
Mar 22 2009

Hydra or the Sea Serpent is the largest and longest constellation in the night sky.  For observers in the mid-northern latitudes this constellation takes more than six hours to completely rise above the eastern horizon.  The gender of the snake is in question because of its name.  It is a male snake in Greco-Roman mythology but because its Latin name is feminine, it is also called the Female Water Snake.

According to one legend, Hydra was the snake that Hercules killed.  This was a difficult task for him because every time he cut off one of the snake’s heads, two more grew back.

Most of the stars in Hydra are fairly dim but one stands out brighter than the rest, Alpha Hydrae (Alphard).   The three stars Alphard, Regulus (in Leo) and Procyon (in Canis Minor) approximate a nice equilateral triangle with Alphard in the southeastern position.  Alphard is the only star in Hydra that has been given a special name.  It is about 85 light-years away and has a magnitude of 2.0.

Hydra has quite a few deep sky objects and we will consider three distinct categories of objects: M48 (open cluster), M68 (globular cluster), and M83 (spiral galaxy).

Located between Procyon (Canis Minor) and Alphard (α Hydrae / Alpha Hydrae and is the brightest star in Hydra, marking the heart of the snake) is M48.  It is a bright open cluster that can be seen on a clear dark night with the naked eye.  It has a visual magnitude of 5.8 and has a diameter of 54 minutes of arc.  It is interesting to note that when Messier recorded this open cluster he made a 4 degree error in its position.  It wasn’t until 200 years later that the error was discovered and its actual position was determined.

M48 has about 50 member stars with a diameter of about 20 light-years.  It has been determined to be about 1.530 light-years distant.

M48

M48

The second object is a globular cluster, M68.  Located just south of Corvus, M68 is a bright globular cluster that, at a magnitude 7.7 and an angular size of about 3 minutes of arc, can easily be seen in a small telescope or pair of binoculars.  It is about 46,000 light-years away and has a diameter of about 100 light-years.

M68 from Hubble

M68 from Hubble

The final object is M83, a spiral galaxy.  It has a fairly large angular size (10′ x 9.5′) for a galaxy and has a brightness of 7.6 magnitude.   This is one of my most favorite galaxies and, under good skies, can be seen with just a pair of binoculars.  To locate this galaxy you are going to have to do a little looking.  It is about 20Âș south of Spica (Virgo) and 10Âș east-southeast of M68.

M83

M83

All three of these objects are binocular objects and well worth the effort to hunt them down.

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