Open Clusters

Stars may sometimes be grouped in numbers of the 100s or the 1000s. Typically, stars that formed in the same nebula and generally stuck together are loosely bound to each other in what is called an open cluster.


An open cluster is just as it sounds. The cluster of stars does not have a defined shaped but does have defined members. These are the characteristics of pen clusters.

• Open clusters consist of younger stars (i.e. bluer stars) with higher metallicities.
• Typically, we find open clusters in the disk of our Milky Way because that is where the younger stars generally are found in our galaxy.
• Open clusters contain anywhere from a few dozen to a few hundred stars and are about 20 light-years across with the main body being about 3 to 4 light years in size.

Open clusters can easily be found in the night sky. In fact, if you look at the constellation Taurus, there is a really bright cluster above the head of Taurus called the Pleiades, or the Seven Sisters.

Pleiades Open Cluster

via APOD



Another example of a open cluster is actually found in the same constellation. The Hyades are found in the head of Taurus and are near the star Aldebaran. Though Aldebaran appears to be in the cluster, it is not because of its age, size, and distance from the others.

Hyades Open Cluster

via APOD

Next time, we will talk about a different stellar cluster, globular clusters.

Novae and Type I Supernovae

Sometimes, when a white dwarf forms, it is in a binary (or multiple) star system. If the companion star is close enough, the white dwarf call pull some of the material off the companion, and accrete it onto the surface of the white dwarf.


When the stellar material hits the hot surface of the white dwarf, the material can fuse quickly and create what is called a nova. Novae are not as energetic as supernovae, but will increase the luminosity greatly. However, the material accreted is rapidly used up and the nova dies down. Nova can occur many times and do have a predictable period that can be measured.


However, if the material on the surface of the white dwarf accretes too fast, the material can increase the mass of the white dwarf to over the Chandrasekhar limit. When this happens, instead of a nova explosion, the white dwarf undergoes a catastrophic collapse. What occurs is a Type I Supernova.


Since the white dwarf mass becomes larger than 1.4 solar masses, the electron degeneracy of the white dwarf cannot overcome the gravity the white dwarf experiences. The white dwarf catastrophically collapses, allowing all the material in the white dwarf to fuse rapidly. The outward explosion from this sudden release of energy completely destroys the white dwarf and companion star. The way astronomers differentiate between Type I Supernovae and Type II Supernovae is the lack of hydrogen lines in Type I. Because the white dwarf has no remaining hydrogen and the hydrogen from the companion star is completely used up in the fusion process, we know that Type I Supernovae can only be created by the sudden collapse of a white dwarf that accretes material from a companion star.

From Wikipedia, OOCalc chart

Looking at the above image, another difference between Type I and Type II supernova is that Type I are typically much brighter in the beginning but fade much quicker.

Exoplanets around Pulsar PSR 1257+12

Back in 1991, the only planets known to astronomers, and to an extent, all of humanity, were the nine known planets of our Solar System. In 1992, that all changed.


A while ago, we learned how planets formed around a star. A stellar nebula begins to rotate, and pockets of the gas and dust collapse and consolidate to form planetessimals. In turn, these planetessimals collide with each other, growing ever bigger, until they reach full-fledged planetary status. The one thing that astronomers were certain of, were that planets would most likely be around main sequence stars and possibly some white dwarfs. However, the first exoplanets discovered were not around either of these objects. They were found around Pulsar PSR 1257+12 by Polish astronomer Aleksander Wolszczan and his team. Why is this strange?


What do we remember about pulsars? Pulsars are rapidly rotating neutron stars, which we know were formed when a medium-high mass star supernovas. And supernovas are extreme events in a high mass star's life. The supernova explosion itself should have obliterated any planets that orbited around that star. So how exactly were planets found around a pulsar?


There are a couple of theories.

1. The planets formed from the supernova remnant. Possible, as long as the remnant did not disperse too quickly
2. The planets were actually lone planets, without a parent star. The planets passed too close to the pulsar and were captured by the pulsar's gravity.
3. The planets somehow survived the supernova event and instead of being obliterated, rode out of the supernova and were pushed farther away.

The most likely theory is the second one, lone planets captured by the pulsar. We do know one thing for sure, however. Though the planets are roughly Earth-sized (A ~ 0.020 Earth masses, B ~ 4.3, and C ~ 3.9), they cannot be hospitable to life. They are all within one AU of the pulsar which means they are bathed in extreme radiation from the pulsar.


A nice thing about pulsars is that they do have extremely precise periods, and any perturbations from a companion planet could easily be measured. The radial velocity technique works well for planets around pulsars (if there are any).

Earth-like Exoplanets

In the last few years, astronomers have been focusing on finding planets considered terrestrial and habitable. Terrestrial planets are Earth-like in terms of composition and size. But what do we mean by habitable?


By definition, a habitable planet is a terrestrial planet in the so-called Goldilocks zone, named after the fairy tale character who found the just-right porridge, chair, and bed in the three bears' house. In this case, what we mean by just right is that the temperature of the planet is just-right, i.e. it is warm enough to have liquid water on the surface.


Why is water important? Water is a strange substance, even though everyone is familiar with water. Water is the only compound in the universe that is at its most dense when it is a liquid, not in its solid state. At 4°C, a given volume of water will be heavier than the same volume of ice at 0°C. Why does this matter? If you know anything about ice fishing or frozen over bodies of water or even drinking ice water, ice floats. Drop any other solid compound in its liquid component, and that solid will sink. This is important because it is theorized that life on Earth began in water. If water were to freeze like any other substance, all bodies of water would freeze solid and life could not survive. So liquid water is important.


Another thing that is important in looking for habitable planets, is that the star that any potential Earth-like planet must have a long evolutionary process. We want to make sure that the planet does not evolve off the main sequence too quickly. We know that life on Earth didn't arise until about 3.5 billion years ago, when the Earth was only a billion years old. However, intelligent life, or maybe just human life is only about 100,000 years old. High-mass stars would not be able to host a planet that could harbor life as those stars would evolve too quickly and supernova within a few million years of forming. For that matter, planets around A-type and early F-type stars would evolve off the main sequence too rapidly for any intelligent life to form.


On the other end, planets around low mass stars, M-type stars, would have to be extremely close to the parent star to be warm enough to have liquid water. However, being so close to the parent star also has problems. Recall that Mercury is tidally locked to the Sun. A planet that close to an M star would also likely be tidally locked which would lead to extreme temperature swings on the light-side and dark-side of the planet. Possible life would be single-celled organisms or those that could survive high temperatures, but intelligent life is unlikely to form on those planets.


So what type of stars do we look for Earth-like planets around? Late F-type, G-type, and early K-type stars are the stars that are stable enough, stay on the main sequence for a few billion years, and hot enough for any Earth-like planets to be a significant distance away from the parent star, but still close enough for liquid water to exist on the surface.

Extra-Solar Planets

For thousands of years, what humans knew of planets consisted of only six objects: Mercury, Venus, Earth, Mars, Jupiter and Saturn. The discoveries of Uranus, Neptune, and Pluto raised that number to nine (until Pluto was demoted). Then in the 1990s, we had an explosion of knowledge: exoplanets.

What is an exoplanet? Exoplanets, sometimes called extra-solar planets, are planets found to be orbiting around stars other than the Sun. They are extremely dim compared to their parent star so can only be inferred by the gravitational influence they exert on their star or the minute dimming of the star as the planet crosses in front.

The first way that a planet can be discovered is if the planetary orbit lies along our line of sight, we use the radial velocity method. Remember that the radial velocity of a star can cause the spectrum of the star shift either to the blue end of the spectrum (blue-shift) or the red end of the spectrum (red-shift). How does this work with a planet orbiting the star?

As seen in the above drawing, if the planet is between the star and us, the planet's small but measurable gravity will pull the star towards us, and we can observe the spectrum of the star to be minutely blue-shifted. If the star is between us and the planet, the planet will pull the star away from us, and the spectrum is red-shifted. Granted, these shifts in either direction are small, but can be measured. From the measurement, the radial velocity on the star can be calculated, and a mass of the planet can be inferred, as well as an idea of the planet's distance from the star.

Another way to determine if there is a planet is orbiting a star is to use astrometry and proper motion. If the orbit of the planet is across our line of sight (i.e. we are looking straight down on the pole of th orbit, this method works best.

In both these cases, the planet pulls the star slightly towards itself and even though the actual motion of the star is small, they can be measured. There are a couple of biases when using these methods including the larger the planetary mass or the closer the planet is to the star, the more the pull that can be observed. This is why when exoplanets were first discovered, they were biased towards what were referred to as hot Jupiters, i.e. large planets close to the parent star. As methods and observational protocols improved, smaller planets and planets farther from the central star began to be discovered.

There are also caveats to finding planets in these ways.

1.      In all likelihood, finding planets that are orbiting along our line of sight, or orbiting across our line of sight are statistically improbable.

2.      Using either method will only give the astronomer a minimum mass of the star.

The radial velocity will only find the velocity that is along our line of sight, i.e. V*sin(i). This will give the mass of the star as M*sin(i). The symbol i is the inclination of the orbit to an orbit across our line of sight. If i is 90°, the orbit lies along our line of sight and sin(i) is 1. The astrometric method will find V*cos(i) and in the same way, the mass of the star as M*cos(i). Combining these two methods will give a pretty good estimate of the planet's mass. However, using astrometry is much more difficult as the angles measured are extremely tiny compared to the actual parallax of the star. Not many planets have been found this way, if any at all.

There is a third way astronomers use to find planets: the transit method, i.e. the planet crossing in front of a star. An astronomer will look at the light-curve* of the star and look for a dip in the intensity of the light.

• Light-curve is the measure of a star's intensity over time. Most stars have a uniform light curve with slight bumps in the curve. Others can vary over time and are called variable stars. More on variable stars later.

Since the planet is so much smaller than the star, the dip is generally tiny and are hard to see in a light-curve because of noise. But if one is found and by looking at how long the dip lasts, astronomers can get an idea of the planet's mass and orbital distance from the star. However, this method is strongly biased towards orbits that are along our line of sight, and cannot be used for orbits that are across our line of sight.

 

Graphic of a Planet transiting its parent star with light curve

Image Credit:

NASA

 

There is a great website that has a lot of information about exoplanets called exoplanets.org. This website has information on all "confirmed" exoplanets from the above methods as well as unconfirmed planets using the Kepler telescope. The Kepler telescope unfortunately does not look at the entire sky, but only a small patch in the constellation Cygnus. Future missions may look at more of the sky and the number of planets discovered will skyrocket.