Making Spectral Energy Distributions

David Maxwell-HD 27524

A spectral energy distribution (SED) for HD 27524, an F5 dwarf star. The chart shows the flux densities of the star, measured in ergs per second per square centimeter at each wavelength in microns. F class stars are considered yellowish-white, and you can see that the flux peaks around the Johnson V (visible) wavelength, as expected. The final scales are logarithmic because the values are exponential, so a log scale makes the values easier to compare.

During our training at Caltech in the summer of 2014, my NITARP students and I learned how to create spectral energy distributions (SEDs) of our target K-giant stars. I wanted to develop a more general lesson plan for my astronomy students and include it on my poster for the American Astronomical Society conference. I decided to have students create SEDs of representative stars for each stellar class.

First, we had to determine which stars were representative. They had to be single stars, so we wouldn’t have source confusion. For each stellar class, there is a standard star. Vega used to be the standard for an A0 star, until astronomers realized that it was a young star with dust still surrounding it. Therefore the readings had too much reddening. Now, they speak of the representative for A0 as an “ideal” Vega. I did some research and came up with two stars for each of the main areas of stars in the Hertzsprung-Russell Diagram, including OBAFGKM and LTY main sequence stars, red giants, post AGB/Wolf-Rayet stars, and white dwarfs.

SIMBAD data on Ross 128

SIMBAD data on Ross 128, a nearby red dwarf star. It includes alternate names, coordinates, proper motion, radial velocity, parallax angle, spectral type, and fluxes (actually magnitudes) at various narrowband wavelengths.

Secondly, we had to look up the magnitudes of these stars at different wavelengths. I wanted to cover the standard Johnson filter wavelengths (UBVRI); the Sloan filters (ugriz) where available; the 2MASS J, H, and K filters; the WISE 1-4 wavelengths; and in some cases, the IRAS 12, 25, 60, and 100 micron data (although I know these are inaccurate from my work at Caltech). There is a repository of stellar data available online called SIMBAD (the Set of Identifications, Measurements, and Bibliography for Astronomical Data), which is ran by CDS out of Strasbourg, France. It contains data on over 8 million extrasolar objects from several catalogs with searchable bibliography.

To use it, one types in the object’s name. They are cross-referenced so that most objects can be found by several names. For example, I choose Ross 128 as my representative red dwarf M-class star. Typing that into the search engine provided the page shown here with all the basic information listed, including alternate names (like many red dwarfs, it is a flare star and so has a V* [variable star] designation), coordinates, proper motion, radial velocity, parallax, spectral type, and photometry fluxes (magnitudes) at various filter wavelengths, in this case the Johnson UBVRI and 2MASS J, H, and Ks. It also has a photograph from the DSS or 2MASS (not very remarkable – it is a red dwarf, after all). But it does have a bluish star close by, so that may interfere with the photometry.

VezieR SED of Ross 128

SED for Ross 128 automatically generated by VizieR. You can roll over the data points to get values, including the flux densities in ergs per second per square centimeter at each wavelength in microns. Here it is showing the IRAS 60 micron data.

There is also a tool called VizieR that will automatically generate an SED for the star and allow narrowing the field of view. In this case, I have zoomed in to 0.2 arcseconds as shown in the chart, and it has a fairly clear Wien’s Law curve that peaks in the red wavelengths (around Johnson R or I) and just to the left of the 2MASS J filter. However, you can see three flux measures at the top of the graph that represent the background blue star. One good thing about VizieR is that you can hover over the dots and it will give information on the mission (in this case IRAS), wavelength (61 microns), frequency (4.85e+3 GHz), electron volts, Janskys, watts per meter squared, and flux in ergs per second per centimeter squared per wavelength.

Mag to Flux Converter-s

Magnitude to Flux converter at the Spitzer Science Center website.

I wanted my students to do the calculations and chart themselves, and use VizieR as a backup and to check their results. This chart also doesn’t have the WISE magnitudes, so I wanted them to use IRSA to get those, then use SIMBAD to record the rest, and convert all of them from magnitudes to flux densities and create their own logarithmic SED charts. We did this, and worked through the MS Excel spreadsheet calculations as we had done at Caltech. I found an online calculator that can convert from magnitudes to flux densities automatically. It is at the Spitzer Science Center website, of all places, and we used it to double-check our answers. Comparing our “by hand” calculations with the automatic calculator produced some inconsistent results, as seen in the diagram of Ross 128 with SIMBAD and WISE data combined. I think this may be because I was using nanometers instead of microns, which would put the results off by a log value of three.

Beta CVn VizieR chart

SED for Beta CVn (Chara), a G0 star, from VezieR. It shows a nice Wien Curve peaking between the Johnson V and R wavelengths. A G star is slightly more yellowish compared with the F5 star above.

To give another example, I have worked through the process again with the G0 V star Beta Canum Venaticorum (Beta CVn), also known as Chara. This star is the “representative” star for the G0 class, and is about 26 light years from Earth. Our sun is a G2 V star, and so is slightly smaller and cooler than Chara. It is considered a top contender for having Earth-like planets with life due to its similarity to our sun, but so far no planets have been detected.

Beta CVn spreadsheet

My spreadsheet for Beta CVn. The flux densities were read from the individual data points in VezieR for this chart. The VezieR chart did not include WISE data.

I looked up the SIMBAD data, then created a VizieR chart. Instead of using the Spitzer converter, I read the flux densities directly from the chart by rolling over the data points, then filled in data from the SIMBAD chart for Johnson U and a few other filters. I went to IRSA and looked up the data in the SDSS, 2MASS, and WISE catalogs, but it did not have the magnitudes for WISE. Apparently the star is too bright to measure without saturating the detector on WISE.

Beta CVn SED-f

Final SED of Beta CVn (Chara) using VezieR data, converted into flux densities by wavelength. It peaks between the V and R filters as expected.

I put the flux densities from VizieR into a spreadsheet and added the formulas, then charted the data. In the case of my chart, it is the log of the wavelength on the horizontal axis and the log of the wavelength times the flux densities per wavelength on the vertical axis. The VizieR chart shows flux densities per frequency. My chart shows an “IRAS tail” in the 60 and 100 micron data, which was typical in our SEDs over the summer. We theorize that the IRAS detector, not having very good spatial resolution, was picking up background infrared flux as well. The Herschel and Spitzer MIPS data for the same wavelengths shows a much more consistent curve. The IRAS data also depends on whether you are using the Faint Source Catalog or the Point Source Catalog.

For the student charts, we had to do some cleanup to move the axes and include the units (log l vs. log (l •F l)). The end results worked well, showing the peak fluxes where expected for the various stellar classes.

Red Supergian SED -f

SED for a red supergiant, KY Cygni. The wavelengths of the Wien curve peak in the K band (deep red) and show a large infrared excess in the WISE bands. This chart was made using the “by hand” conversion method in an Excel spreadsheet.

As long as you use a consistent method of calculating throughout (such as using only VezieR fluxes, or only the Spitzer converter, or using only the “by hand” calculations), the results work out well. For the SED of KY Cygni, a red supergiant, I used only the “by hand” method. The Johnson V, 2MASS JHK, and WISE data all line up as they should with a large infrared excess (what one would expect of a red supergiant). I am attaching John Gibb’s worksheet for doing the conversions from magnitudes to flux densities by wavelength here. He created it for our summer workshop at Caltech.

Making SEDs starting with magnitudes using Excel

I am including some of the final charts created by my students. As previously noted here, a lot can be understood about a star from an SED, including whether it has an infrared excess from a shroud of dust or gas surrounding it (such as Vega), what type of star it is, etc. Because SEDs are used frequently by astronomers, I wanted my students to learn how to make them as well. We were getting close to the end of the semester and I didn’t get the full lesson plan worked up, but most of the students were successful in creating the charts.

WISE data -KY Cygni

WISE data for KY Cygni, a red supergiant, in the IRSA database at Caltech. The magnitudes for the WISE 1-4 images are listed in the green line below. You can see that the detector is saturated in the WISE 3 image.

I still hope to create lesson plans on doing photometry based on my BYU RET training over the summer of 2014. At the very least, however, I have shown to my satisfaction that high school students are capable of doing some fairly high-end astrophysics data analysis using readily available free and public sources. I gave the students some feedback forms for each of these three lesson plans, and the results and their test scores showed they had a good understanding of the concepts involved and what the data meant, even if they could have used more time on the exact procedures. I felt I had what I needed to finish my poster for the upcoming American Astronomical Society conference.

Brown Dwarf SED

An SED for a brown dwarf. Notice that the overall flux density values are very low, and that they are still going up beyond the 2MASS JHK data points at the right. Although we do not have WISE data here, the fluxes probably peak in the infrared. This star gives off very little visible light.


About davidvblack

I teach courses in multimedia, 3D animation, Earth science, physics, biology, 8th grade science, chemistry, astronomy, engineering design, STEAM, and computer science in Utah. I've won numerous awards as an educator and am a frequent presenter at state and national educator conferences. I am part of the Teachers for Global Classrooms program through the U.S. Department of State and traveled to Indonesia in the summer of 2017 as an education ambassador. I learned of the Indonesian education system and taught classes in astronomy and chemistry at a high school near Banjarmasin in southern Borneo. I am passionate about STEAM education (Science, Technology, Engineering, Arts, and Mathematics); science history; photography; graphic design; 3D animation; and video production. This Spaced-Out Classroom blog is for sharing lessons and activities my students have done in astronomy. The Elements Unearthed project ( will combine my interests to document the discovery, history, sources, uses, mining, refining, and hazards of the chemical elements.
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