This week I discuss types of supernovae, specifically relating to the scenario where “Hydrogen lines are prominent in Type II supernovae but absent in Type Ia. Type Ia supernovae decline gradually for more than a year, whereas Type II supernovae alternate between periods of steep and gradual declines in brightness. Type II light curves therefore have a step-like appearance. Explain!”
The question I really wanted to ask is ‘What happened to Type I or Ib?’ and the answer to that question was easily found in this chart:
Just in time for Halloween, my topic this week focuses on electron degeneracy pressure specifically to delve into how “A degenerate gas does not expand when the temperature increases as an ordinary gas does.”
In 1923, Arthur Stanley Eddington derived a formula to relate the luminosity of a star to its mass, and in the same year correctly interpreted high-density, white dwarf stars as being formed of matter so dense that atomic electrons have collapsed from their orbits, a substance we now call degenerate matter. (Levy, p. 116)
Young giant stars of a certain size, between .4 and 2 solar masses, have helium rich cores squeezed by gravitational force into a crystal-like solid. At these pressures, the atoms become completely ionized, separately into nuclei and electrons and are so closely crowded together that become influenced by the Pauli exclusion principle phenomenon. Two identical particles are not allowed to exist in the same place and time. As the electrons are pressed closer and closer together, the exclusion principle forces many of them to move faster and faster so they do not become ‘identical’ (meaning occupying the same space and moving with the same speed of adjacent electrons) and consequently this motion increases the repulsion between the electrons. This state provides pressure in the core preventing it from collapsing and is referred to by astronomers as degeneracy. Thus, low-mass giants with helium-rich cores are supported by electron degeneracy pressure. (Comins, p. 335)
This week I want to discuss “What might cause the closer of two identical stars to appear dimmer than the farther one?”
Apparent Magnitude: A measurement of the brightness of stars without regard to their distance from Earth.
The scale in use today starts with the star Vega and an apparent magnitude of 0.0
Objects brighter than Vega are assigned negative numbers. For example. Sirius, the night’s brightest star, has an apparent magnitude of -1.44
The scale was extended to include the dimmest stars visible through binoculars and telescopes. For example, a pair of binoculars can see stars with an apparent magnitude of +10
Ignoring distance for a moment, all other things being equal, the closer of two identical stars will appear brighter (have a smaller apparent magnitude) to us than the more distant star. When we account for the difference in distance, we use either or two measurements: absolute magnitude and luminosity.
I’ve reached the halfway point through my Introduction to Astronomy class. This week marks the eighth week of fifteen, sixteen if you count the first week where we just spent time getting to know each other and exploring the textbook and getting the lab software, Starry Night, installed and licensed. Last week, we reached the outer limits in the Kuiper Belt and Oort Cloud of our solar system where only comets and Voyagers I and II have ventured. Now we’ve snapped back to study our closest star, Sol, or more commonly just the Sun. My topic for discussion responds to the following question:
Why is the solar cycle said to have a period of 22 years, even though the sunspot cycle is only 11 years long?
Some surface features on our active Sun vary periodically in an eleven year cycle. The Sun’s magnetic fields which cause the surface changes vary over a twenty-two year cycle. The relatively cool and slightly darker regions, commonly called sunspots, are produced by local concentrations of the Sun’s magnetic field piercing the photosphere. The latitude and number of sunspots on average vary during the same eleven year cycle. But the hemisphere where the Sun’s north magnetic pole anchors during one eleven year cycle will have south magnetic poles during the next. Because it takes a full twenty-two years for the magnetic poles to return to their original orientation astronomers refer to the entire solar cycle. The magnetic dynamo model posits that many transient features of the solar cycle are caused by the effect of differential rotation and convection on the Sun’s magnetic field. The Sun’s differential rotation (different speeds at different latitudes) causes its magnetic field to become increasingly stretched like a rubber band. The bands can’t break so they periodically untangle themselves with the help of trapped gases which leak out (sunspot) and gradually settle back under the photosphere, when the sunspot disappears. The most recent reversal of the Sun’s magnetic field occurred in 2013. We are currently at the tale end of Solar Cycle 24. (Comins, 2015, p. 272-83)
All last week, I looked forward to the weekend as a chance to get some astronomical observing accomplished. The weather forecast seemed too good to be true: Sunny and clear, highs in the mid 70s and lows in the 50s, with dew points in the upper 40s and lower 50s. My astronomy club hosted a club star party, but I did not want to lug the scope to Louisburg and share the observing grounds with a previously scheduled private party. Continue reading “Backyard Observing”
Autumn arrived mid-week here in the Heart of America, but you wouldn’t have known it by looking at the weather forecast: Mid 90s and moderately high humidity. Also with the change of the seasons, I retired my FitBit Charge (or rather it retired itself by falling apart) and upgraded to a Samsung Gear Fit2. The new fitness tracker is spurring me on to be more active, although my sleep pattern hasn’t improved much. I can safely blame work (10 pm to 4 am conference call on a Saturday night/Sunday morning) and astronomy, which requires, well, dark skies, for my reduced snooze time.
Tomorrow, just after six o’clock in the morning and just as the sun is rising, we’ll experience the first full moon to occur on Christmas Day since 1977. I wasn’t even in high school yet in 1977 (although my husband was already in college by then). If you miss opening this Christmas present, you won’t get another chance until 2034 (by which time I should be retired).
Other astronomical items of note this holiday week include:
On the 4th day of Christmas (Monday that is), Mercury reaches its peak distance from the sun 30 minutes after sunset in the southwest.
On the 5th day of Christmas (Tuesday), Saturn continues its return from behind the Sun. Look to the southeast in the pre-dawn morning time.
On the 6th day of Christmas (Wednesday), look up and south to spy the Seven Sisters (aka as the Pleiades)
On the 8th day of Christmas (Happy New Year!), use binoculars to find Comet Catalina rising close to Arcturus (a very bright star) around midnight and continue to rise high in the southeast until dawn twilight.
On the 9th day of Christmas (Saturday, January 2, 2016) the Earth reaches its closest point to the Sun (at the start of Winter no less)
On the last day of Christmas (Twelfth Night) at 10 p.m. EST, Pluto hides behind the Sun.
The day after the Kansas City metro area got nearly a foot of snow dumped on it, I ventured out to return to work. Most of the local schools and some businesses remained closed that day, but not my employer or the employer of one of my other vanpool riders. On the commute home, I enjoyed watching some sun dogs playing around the sinking sun. My smartphone camera just doesn’t do them justice: