Exploding Stars Teacher Guide

Introduction

Vera C. Rubin Observatory will detect millions of faint supernovae in galaxies at farther away than ever before. Type Ia supernovae are of particular importance because they can be used to measure distances in the nearby Universe, and accurately map the locations of galaxies.

Students learn how to differentiate supernova light curves that result from the explosion of high mass main sequence stars vs. supernovae that originate from white dwarf stars in a binary system. Students then use light curve data from Type Ia supernovae along with the distance modulus to determine the distances to host galaxies.

Learning Outcomes

• Students interpret light curves to determine the type of supernovae represented by the curve, and their progenitor stars.
• Students can explain why Type Ia supernovae can be used to measure distances in space but Type IIp supernovae cannot.
• Students estimate distances to galaxies by using the apparent and absolute magnitudes of Type Ia supernovae.

Prerequisite Concepts

• Students should be familiar with apparent and absolute magnitudes and the magnitude scale.
• Students should know that stars and supernovae create elements through the process of nuclear fusion.
• Students should know about the two most common types of stellar explosions (supernovae) and how they originate from either a high mass main sequence star or from a binary system containing a white dwarf.
• Students should have been introduced to the astronomical concept of a “standard candle”- a certain type of object with a consistent peak luminosity (in this case, a Type Ia supernova) that can be used to measure distances in space.

Where This Fits In Your Teaching

• cosmological redshift
• Hubble's law
• Doppler shift
• distance ladder
• standard candles
• stellar evolution
• variable stars
• apparent and absolute magnitude
• cosmology
• expansion of the Universe

NGSS Storylines

• What kinds of stars can explode to form a supernova?
• What kind of supernovae can be used to measure distances, and why only that type?
• How can you identify different types of supernovae?
• What information is needed to calculate a distance to a supernova, and how do you do it?
• What fusion changes happen in the interior of a star to cause a supernova?
• Will the Sun blow up one day?
  

Suggested investigations which could come BEFORE this one:

Unlocking the Distances to Galaxies

A Window to the Stars

Suggested investigations which could come AFTER this one:

Expanding Universe

Exploring the Observable Universe

See Related Rubin Observatory Investigations for more details.

Investigation Timing

Online component: 45 - 75 minutes.

Standards

Three-dimensional lesson summary

Students fit templates to light curves of supernovae to determine the type of star that exploded. They make peak magnitude measurements from light curves of several newly-discovered supernovae, and use a mathematical function to calculate the distances to the galaxies which contain the supernovae.

Building towards:

HS-ESS1-1 Develop a model based on evidence to illustrate the life span of the sun and the role of nuclear fusion in the sun’s core to release energy that eventually reaches Earth in the form of radiation.

HS-ESS1-2 Construct an explanation of the Big Bang theory based on astronomical evidence of light spectra, motion of distant galaxies, and composition of matter in the universe.

HS-ESS1-3 Communicate scientific ideas about the way stars, over their life cycle, produce elements.

Science Literacy and Critical Thinking Skills

• Analyzing and interpreting data
• Using mathematical and computational thinking

Background

This investigation uses only two common types of supernovae: Type Ia (from white dwarfs) and the most common of all, Type IIp (from high mass main sequence stars). There are many other types, distinguished by their spectral line emissions. Type IIp supernovae outnumber Type Ia by a factor of 2:1. But because Type Ia supernovae are more luminous, they can be detected at greater distances, so observations reveal about equal numbers of both types.

Only certain types of supernovae can be used to measure distances. Type Ia supernovae arise from white dwarf stars in binary systems, in which the white dwarf has accreted matter from its partner star. There is a mass limit for stable white dwarfs, called the Chandrasekhar limit, which is about 1.4 solar masses. If the mass exceeds this number, the star reignites nuclear fusion and becomes a supernova. Since all white dwarfs explode at the same threshold of mass, the peak brightness from each one’s explosion is always the same, and therefore the supernova can be used as a standard candle. This is not true for other types of supernovae, collectively known as core-collapse supernovae, that occur at the end of the lives of massive stars, because these stars can vary in size and mass.

Type Ia supernovae can be used to map the location of galaxies that we may not even be able to see. Some of these galaxies in your data set may be so faint that you will only see the light from the supernova, which when at peak luminosity far outshines the light from its host galaxy.

Openstax Astronomy textbook links:
The explosion of massive stars
The explosion of white dwarf stars in binary systems
Standard candles and Type Ia supernova

Links to Videos

These resources are from a teacher workshop on this investigation.

Video: Federica Bianco, "Explosions in our Data"
Speaker slides

Teacher Notes

1. This investigation simplifies the distance modulus equation in two ways: First, a peak light curve absolute magnitude of -19.4 is assumed. (There is a ±0.5 apparent magnitude variation in the peak of some of the more distant Type Ia supernovae.) Second, the factor for dust extinction is eliminated. This may cause the calculated distance to be closer than its actual distance.

2. A correction for time dilation effects due to cosmological expansion has been pre-applied. This adjusts the spacing on the time axis of the light curve plots so that it is more accurate for the more distant supernovae.

3. Although Type Ia supernovae are referred to as “standard candles,” it is not accurate to say that the shapes of all Type Ia light curves are exactly the same. There is a relationship between the peak luminosity and the rate of decline of the supernova. Different methods have been developed for fitting the light curve data to a template or a series of model light curves that correct for these luminosity differences—this technique is known as standardizing the light curve. See this link for more information.

   In this investigation, students use a single Type Ia template and can stretch and compress it to mimic the suite of templates used by astronomers. This more intuitive interaction reinforces the idea that there is a practice of fitting the data to a type of template in order to standardize it. The limitation of this procedure is that the fits will not be as exact without access to the full suite of templates.
   

4. The low quality of the galaxy/supernova images results from each image being a short, single exposure over a limited range of wavelengths, which is sufficient for supernova detection. The image quality is not related to the distance of the galaxy. Colorful and well-focused galaxy images that you may be more familiar with are produced by combining many exposures of the same image to reduce image noise and accumulate more light, and by imaging the galaxy over a wider range of wavelengths.

Common Student Ideas

1. Most or all stars blow up at the end of their lifetimes. Our Sun will one day blow up.

   Bridge to learning: Not only is this a common misconception, but some students can confuse the red giant stage of the Sun with the Sun “blowing up”, because the Sun’s red giant phase is often described as “engulfing the Earth.”

   Explain that for solitary main sequence stars, mass alone controls whether or not they will explode as a Type IIp supernova. Main sequence stars with less than eight solar masses will not form a supernova.

   What about the Sun exploding once it becomes a white dwarf?

   A Type Ia supernova can happen only when a white dwarf is a member of a binary star system, and since the Sun does not have a binary star partner, it will never explode as a Type Ia supernova.
   
   

Common Student Questions

1. What causes a star to blow up?

   There are two different mechanisms. White dwarfs that accumulate too much material from their partner star heat up so much that they begin to fuse carbon, creating a thermal runaway fusion reaction which leads to a Type Ia supernova. Another possible scenario is the merger of two white dwarfs in a binary system.

   Massive stars undergo core collapse when they reach a point where the forces generated by fusion are unable to overcome gravitational force. This results in the violent outward explosion of the outer layers of the star, producing what’s called a core collapse supernova. Type IIp supernovae are the example we use in this investigation, but there are other types (e.g., Type Ib and Type IIL). Type Ia is the only class of supernovae not caused by core collapse.
   

   http://astronomy.swin.edu.au/cosmos/c/core-collapse

2. What other kinds of supernovae are there?

   Other kinds of supernovae are classified based on slight variations in the shapes of their light curves, and differences in which elements are observed in their spectrum.

   Some very energetic supernovae are called hypernovae.
   

   https://en.wikipedia.org/wiki/Supernova#Classification

   https://upload.wikimedia.org/wikipedia/commons/e/e0/Comparative_supernova_type_light_curves.png
   

3. Could the supernova of a nearby star destroy the Earth?

   Probably not. The average “safe distance” from a supernova is estimated to be somewhere between 50 -100 light years, and at present there are no known stars within that distance that could one day form a supernova.

   http://earthsky.org/astronomy-essentials/supernove-distance

Differentiation

Students use an interactive tool to examine how changes in supernova brightness correspond to changes in magnitude represented on a light curve graph. Students receive feedback when attempting to identify the location of a supernova in a galaxy. The interactive light curve plotting tool comes with data point error bars and other representational assists to help students analyze and interpret their curves and identify key points and their values.

Visuals, templates, and narrative descriptions provide different ways for students to recognize patterns in light curves and categorize types of supernovae.

Students use their supernovae observations to calculate the distances to galaxies, and then connect the galaxy locations by matching to areas within the local Universe, to build a sense of scale. They compare their results to see how far away supernovae observations can be used to measure distances.

An assessment task may be assigned in place of or in addition to the summary, "Putting it all Together". Students can use class data to construct a three dimensional model with Earth at the center to show the locations of their galaxies in the local Universe by using the galaxy right ascension and declination as x, y coordinates in space and the calculated distance as z. Alternately, these values could be plotted using a 3D graphing tool or as a coding exercise.

To add a higher level of challenge, refer to the “Ideas for Further Study” at the end of this guide.

Related Rubin Observatory Investigations

• In the Unlocking the Distances to Galaxies investigation, students will analyze light curves from Cepheid variable stars and the period-luminosity relationship to determine distances to other galaxies.
  
  
• In A Window to the Stars, students use H-R Diagrams and histograms to explore the properties of different types of stars in their evolutionary sequence.

• The Expanding Universe investigation uses Type Ia supernovae to measure the distance to galaxies, and redshift data to measure their velocity. Students verify that the Universe is expanding and that the rate of this expansion has changed over time.

• In Exploring the Observable Universe students use photometric redshift data to evaluate the distance to galaxies, to verify that the Universe is expanding, and to see how the large scale structure of the Universe has changed over time.

Ideas for Further Study

• Classify a large number of supernovae to determine their statistical rate of occurrence (Type Ia vs. Type IIp).

• Supernovae used in this investigation occur in galaxies external to the Milky Way. Students can explore which supernovae occur in which types of galaxies, and for the case of spiral galaxies, where in the galaxy they originate (arms, core, halo).

• Find the common set of galaxies in Rubin Observatory data that have both a Type Ia supernova and a Cepheid variable star, and calculate the distance to the galaxies using the two techniques. How much in agreement are the values? Do the values get more different from each other with increasing distances?

• There are other subclassifications of Type I and II supernovae. In the future it may be possible to add additional datasets and light curve templates to identify more subclassifications.

• Compare the distances obtained from Type Ia supernovae to the distance obtained from galaxy redshifts. Are they in close agreement? Do the values get more different from each other with increasing distances?

• Use the galaxy right ascension and declination as x, y coordinates in space and the calculated distance as z. Construct a three dimensional model (with Earth at the center) to show the galaxy locations in the local Universe. Alternatively, plot the values using a 3-d graphing tool. This could be done as a coding exercise as well.