Now that summer is here and the evening weather is great, there is almost no reason not to go out and take in the night sky. The trade off, however, is that it doesn’t get completely dark until almost 10pm, but some of the most beautiful constellations are found in the summer sky. This month I will cover another zodiacal constellation before touching on an unremarkable constellation that is just too large not to be included. Then I will move on to three constellations that contain the three bright stars of The Summer Triangle. Interestingly, all three were among the 48 constellations catalogued in the 2nd century BC by Ptolemy.
Hercules and Virgo from last month’s Scope Out are jumping off points for finding this month’s constellations. A planosphere or smartphone app like Google Sky Map can help determine where to look to find the constellation, and a Wikipedia link is provided to further aid in identifying each constellation’s appearance.
Libra (The Scales)
Libra is our zodiacal constellation for this month. It is a small constellation of rather dim stars, located to the east, or left of Virgo. As it is located on the ecliptic, but south of the celestial equator, it never rises very far above the horizon for observers in the northern hemisphere. These two factors combine to make this a difficult constellation to observe. I would contend that the most notable fact about this constellation is that it is a zodiacal constellation.
Ophiuchus (Serpent Bearer )
Ophiuchus is located on the celestial equator just south of (or below) Hercules. This is one of the largest constellations in the sky. This nondescript constellation will require a little work to find the constellation and locate its stars, because they are not very bright and they are are widely scattered. It is found just slightly east and south (to the left and below) Hercules.
Aquila (The Eagle)
Aquila is a fairly easily recognized constellation of medium to bright stars. It is a pretty constellation that easily evokes an image of a soaring eagle, and it contributes Altair to the Summer Triangle. It can be found to the east (left) of Hercules.
Lyra (The Lyre)
This is a small, but pretty constellation that contributes the star Vega to the Summer Triangle. Lyra is located north and east (above and to the left) of Hercules, and all of its stars can be readily picked out, even in our light-polluted skies. Lyra contains one on the most interesting Messier objects, the Ring Nebula (M57), which is located half-way between the two stars of the parallelogram asterism most distant from Vega. I have been able to see it pretty well with my 4″ apochromatic refractor, but it might be a stretch for less capable telescopes.
Cygnus (The Swan)
The last of the three constellations containing the three Summer Triangle stars is Cygnus. It is located east (to the left) of Lyra. Its brightest, and its Summer Triangle star, is Deneb. A gracefully flying swan can be easily imagined while taking in this constellation. Cygnus is also known as the Northern Cross.
SOLAR SYSTEM OBJECTS
As the Sun has passed Summer Solstice, the days will begin to shorten in very small increments. By month’s end, the Sun rises 21 minutes later and sets 17 minutes earlier than it did at the beginning of the month. This means that by month’s end, a day is only 38 minutes shorter than it was at the beginning of the month. The reason for this slow rate of change is that the Sun is on a relatively horizontal portion of the ecliptic in July, meaning that the rate of change for each day’s length is quite low. Compare in the star chart below the slope of the ecliptic near the Summer Solstice (July 1st) with the slope of the ecliptic near the Autumnal Equinox (Sept 21st). Note that although the slope of both arrows is downward (negative slope), the downward slope is steeper in September. This means that the rate of change is higher in September, and the days will shorten more quickly than now. We’ll monitor the rate of change over the next few months to observe this phenomena for ourselves.
|July 5||First Quarter|
|Very close conjunction with Mars|
|July 7||Very close conjunction with Saturn|
|July 12||Full Moon|
|July 19||Last Quarter|
|July 24||Conjunction with Venus|
|July 26||New Moon|
Venus rises before sunrise and remains a morning star throughout the month. It is the brightest object in the eastern sky in the pre-dawn hours, and is still situated above and to the right of the sunrise point. It will slowly decrease in brightness as its elongation from the Sun continues to decrease.
Mars remains in Virgo, and is already well above the horizon at sunset.
Saturn remains in Libra, and is well above the horizon in the east at sunset.
DEEP SPACE OBJECTS (DSOs)
As I mentioned last month, Scope Out has examined nebulae, star clusters and galaxies in this section. This month I will focus on a very special galaxy, the Milky Way Galaxy, the one that contains our solar system. Milky Way also refers to the diffuse band of light that arcs across the summer sky from Sagittarius in the southeast to Cassiopeia in northwest. It’s name is derived from its “spilled milk” appearance.
I recall seeing the Milky Way often as a child growing up in semi-rural Georgia, but I have not been able to see it where I presently live because the light-polluted sky is brighter than the Milky Way. It is best seen as far away from city lights as possible, and on a Moonless night.
What I hope to do in my treatment of the Milky Way is relate the band of light that is seen stretched across they sky to the structure of the spiral galaxy that the Milky Way actually is. In order to make this leap, I must do three things: 1) describe a spiral galaxy’s structure and appearance by using far away galaxies and a Frisbee as proxies, 2) describe the Solar System’s location within the Milky Way Galaxy, and 3) take an imaginary journey to our location inside of the Milky Way Galaxy.
Before proceeding, it will be helpful for us to reacquaint ourselves with light and time, and their relationship to distance. Light travels in photons that are either emitted from hot objects like light bulb filaments, or reflected off of cooler objects, like the walls. The Sun and Moon are also emission and reflection objects, respectively. Photons travel from their source to our eye or camera sensor at, well, the speed of light. We can express this speed in terms with which we are already accustomed, like miles per hour (mph). Since light travels at 186,282 miles per second, that equates to 670.6 million mph. That is incredibly fast! Since mph has a distance component (miles), we can use the speed of light, or the speed of anything else, to measure distance. If something is 670.6 million miles away, we can also say that it is one light-hour away.
The amount of time involved essentially becomes the yardstick. The Moon is about two light-seconds away. The Sun is eight light-minutes away. A light-hour does not get us all the way to Saturn, and the closest star is four light-years away. Galactic sizes are on the scale of hundreds of thousands of light-years, and intergalactic distances are on the scale of millions and even billions of light-years. There’s nothing like the distances across the vast emptiness of space to make light seem to flow like molasses.
An implication of light’s incredibly slow speed is that our eyes are essentially time machines. The point at which Saturn appears in the sky is actually where it was over an hour ago. Similarly, The Andromeda Galaxy, our closest galactic neighbor, is seen as it was 2.5 million years ago. A supernova detected today in a galaxy located 11 million light-years away actually happened 11 million years ago. With your mind sufficiently bent, let’s proceed on our three-step journey to “see” our place in the galaxy.
1. Structure and Appearance of a Spiral Galaxy
We can learn much about the structure of a spiral galaxy by observing neighboring galaxies that we can see from various perspectives. We learn from these observations that a typical spiral galaxy is a disk that is thin at its edges, and becomes thicker at points toward the center. There are essentially three perspectives that matter: face-on, edge-on, and in any perspective between the two.
The first perspective to consider is that of a galaxy viewed edge-on. From this perspective, a galaxy’s profile is thin at its edges and it grows increasingly thicker toward the center, and there is a noticeable bulge at the galaxy’s center. The line of sight is through the longest dimension of the disk, so billions of stars contribute to a distant galaxy’s faint glow that we perceive through an amateur telescope. These features can be seen in NGC 4565, which is an example of an edge-on galaxy located about 43 million light-years away. Zoom in on the image at the link and notice that there are brighter areas were masses of stars can be seen, and other areas where the stars are obscured by dust lanes. A Frisbee flying toward you at eye level would present a profile similar to that of an edge-on galaxy.
The second perspective to consider is that of a galaxy viewed face-on. It will appear as a near perfect circle with spiral arms of dust and stars. Generally, a face-on galaxy will be brighter in its center, because stars are more densely aggregated about the galactic center, and because the disk is thickest at the center bulge. Messier M74, located about 32 million light-years away, is an example of a galaxy viewed face-on. Click on the image and zoom in, and again notice the bands of stars and bands of dust. A Frisbee seen lying on the ground has a similar circular profile.
A third perspective would be a view from any perspective other than edge-on or face-on. The profile of such a galaxy is elliptical, with the flatness of the ellipse being related to the offset angle. Messier M31 (Andromeda Galaxy) is an example that is located 2.5 million light-years away, which makes it the Milky Way’s closest galactic neighbor. Our Earth-bound view of M31 is about half way between edge-on and face-on perspectives. Zoom in on the image at the link to see all of the features previously described in the face-on and edge-on perspectives of galaxies.
Even though a galaxy contains billions of stars, it is important to keep in mind that a galaxy is mostly vast tracts of empty space. Our galaxy is so vast in size that we can only see a tiny bit of it. It is believed to be about 120,000 light-years in diameter and it is believed to contain about 250 billion stars. In contrast, the farthest star that we can see with the unaided eye is a very bright one that is about 4,000 light-years away, but something on the order of 500 light-years is typical of the most distant stars that we can easily see from Earth. Within this radius, that are only about 5,000 individually resolvable stars that can be seen by the unaided eye. When stars are viewed at greater distances, they are too dim to resolve individually and the apparent distance between stars grows smaller. As a result, when looking toward the center of our galaxy where stars are more densely packed, we see literally billions of stars that are too dim to be individually resolved, but the combined effect of these billions of stars is that they are perceived as a diffuse patches of light.
2. Location of the Solar System.
The solar system is located within the flat disk of the Milky Way Galaxy, and is about 27,000 light-years from its center. Since The Milky Way Galaxy’s diameter is 120,000 light-years, its radius is 60,000 light years. Thus, we are located just inside a point about half way between the center of the galaxy and its outer edge. Since there are more stars concentrated in the galaxy’s center we will see more stars when gazing in that direction than when we are gazing outward toward the nearest edge where stars are more sparsely spread. To mark a corollary of our location on say a 10″ Frisbee, one could put a star sticker about 4 1/2 inches from its center. More precisely, our location is beneath the star, somewhere about half way between the top and bottom of the Frisbee’s cross section.
3. A Journey to the Solar System
Let’s begin our imaginary journey to the Solar System’s location within the Milky Way Galaxy. In order to scale this journey down to something we can more easily understand, we will once again employ the Frisbee proxy. To orient ourselves, lets first stand near the Frisbee and look down upon it observing a profile similar to the face-on view of the galaxy. Note the location of the star sticker that we put on the Frisbee to mark our notional location. Now place yourself at a point where you are on a line drawn outward from the center of the Frisbee through the star sticker, and now take three or four steps away from the Frisbee in the direction of this line, and place your eyes down at ground level so that the Frisbee is viewed edge-on, and imagine that the Frisbee is our own Milky Way Galaxy. Initially, the view of the galaxy will appear similar to the example edge-on galaxy, NGC 4565, that we examined earlier. Please have this image fresh in your mind before we start moving, because this is how our galaxy appears from our initial vantage point.
At long last we begin our journey, and as we approach the Milky Way Galaxy, its appearance does not change much from the edge on view of NGC 4565. It will only grow proportionately wider and thicker in size as we move closer to the Milky Way’s center. At about 300 light-years from the disk’s edge, individual stars immediately ahead become individually resolvable. A glance in the direction away from the Milky Way is eerily devoid of stars . Once inside the edge, the nearest stars will begin to “fly” past us in a manner similar to what is seen on certain Windows screen savers or on Star Trek. Ahead of us we still see the patchy fog of the dense galactic core as the backdrop for the foreground stars, and behind us we can see some stars, but against a completely dark backdrop. Finally, we reach the Solar System and stop. Because an overwhelming majority of all the Milky Way’s stars are still ahead of us in its dense center, the only significant change in the galaxy’s appearance since we began our journey is that it grew wider and thicker as we moved closer.
Now that we understand a spiral galaxy’s structure and appearance, know our location in the Milky Way Galaxy, and have seen what happens as we fly into the galaxy from its edge, let’s now examine a fish eye photographic image of the Milky Way on the National Geographic Web site. The Milky Way appears as an arch in this image due to fisheye distortion, so to get a true perspective, imagine that the arch is straightened out. Compare this Milky Way image with the edge-on image of NGC 4565 previously viewed. Can you see…bright patches of unresolved stars? …dark dust lanes? …a flattened, edge-on disk? …a bright central bulge? Armed with enough information, it is possible to gaze upon the Milky Way as seen in the summer sky from Earth and understand that it is actually the disk of the Milky Way Galaxy!
Aphelion – July 24
The Earth reaches aphelion (Greek apo [away from] + Helios [Greek god of the Sun]) on July 24th. Aphelion is the point in the Earth’s orbit that is farthest, or 94,555,000 miles from the Sun. Relatedly, perihelion (Greek peri [around] + Helios [Greek god of the Sun] is the point in the Earth’s orbit that is closest to the Sun, or 91,445,000 miles from the Sun.
Aphelion and perihelion apply not only to the Earth’s orbit about the Sun, but any object that orbits the Sun to include all of the planets, asteroids, comets, and even man-made satellites in solar orbit. Apogee and perigee are similar terms for objects that orbit the Earth, which are either the Moon or man-made satellites.
Aphelion and perihelion points are not only opposites that describe the Earth’s farthest and closest distance to the Sun during each annual orbit, these two points are on opposite sides of the Sun. Aphelion occurs July 24th and perihelion occurs on January 4th. Although the aphelion and perihelion points are on exactly opposite sides of the Sun from one another, the dates are not exactly six months apart. This is because the Earth moves more slowly during the perihelion to aphelion (up hill) part of its orbit, and faster while during the aphelion to perihelion (down hill) part of its orbit.
We learned in grade school that the Earth’s average distance to the Sun is 93 million miles. Those who remember that fact at all are likely, if asked, to omit “average,” and state that the Sun is 93 million miles away. If the Earth varies in distance to the Sun from 94.5 to 91.4 million miles over the course of each orbit, then that accounts for the average figure of 93 million miles that is often cited.
Let’s explore why the Earth’s distance to the Sun varies, and what the implications are of that variance. If the Earth were in a perfectly circular orbit, then its distance from the Sun would not vary. In this case, 93 million miles might be the constant distance to the Sun throughout each annual orbit. Like the orbits of most objects around a parent body, the Earth’s orbit is not circular, but is elliptical. An ellipse resembles a stretched or flattened circle, and an object following this type of orbit will vary between its closest and farthest points once per orbit.
The Earth’s elliptical orbit has implications for solar eclipses. Both the Sun and the Moon are said to be about 1/2 degree in angular diameter as observed from Earth. This is because the actual diameter of both or coincidentally 400 times their diameter, and it accounts for the near-perfect fit when the Moon barely covers the face of the Sun during solar eclipses. The Sun’s apparent size when seen from Earth varies inversely with respect to Earth-Sun distance as the Earth moves from aphelion to perihelion. The Sun appears smaller when viewed from Earth at aphelion, and larger at perihelion. Similarly, the Moon appears smaller at apogee and larger at perigee. Consider the effect if the Earth were at perihelion and the Moon were at perigee at the time of a solar eclipse. The Sun would be at its smallest possible apparent diameter and the Moon would be at its largest possible apparent diameter. The result would be a total Solar eclipse where the larger Moon completely obscures the face of the smaller Sun. Now consider the opposite case. The smallest possible Moon at apogee would not be able to completely obscure the face of the largest possible Sun at perihelion. The result would be an annular Solar eclipse where the Sun would appear as a ring around the Moon.
Conjunction of Ceres and Vesta – July 4th and 5th
The two largest asteroids, Ceres and Vesta will reach a very close conjunction on July 4th and 5th. On these two nights they will appear within 10 arc minutes of one another. As points of reference, there are 60 arc minutes in a degree, and the Moon’s apparent diameter is about 30 arc minutes, or as previously stated, 1/2 degree in diameter. These two bodies are not visible with the unaided eye, but can be seen with binoculars or a modest telescope. The conjunction will occur in Virgo, but do your research to know exactly where to look, and print out a chart with stars down to 9th magnitude.
© 2014 James R. Johnson.