Object 118: SN 2005cf

Podcast release date: 19 February 2024

Right ascension: 15:21:32.2


Epoch: ICRS

Constellation: Libra

Corresponding Earth location: Over 1000 km east-northeast of the Marquesas Islands

SN 2005cf is a supernova that appeared on May 28, 2005 [1], in the constellation Libra. As a quick reference, supernovae are typically given names that include the year in which the supernovae were seen and one or more letters indicating the chronological order in which those supernovae were seen, with A being the first one seen in any year, B being the second, and so on through Z. The 27th supernova in any year gets labelled aa, then the next is ab, and so on through az, and then this process repeats starting with ba and then ca and so on. This means that SN 2005cf was the 84th supernova seen in 2005.

A combination of technological advances and organizational activities have meant that, in this century, many supernova monitoring programs are in place and are going to catch supernovae soon after they appear, and those supernovae can potentially be observed very extensively at multiple wavelengths. With SN 2005cf, we know that it appeared in a structure of stars and gas joining the galaxies MCG-01-39-003 and NGC 5917 [2]. This structure is called a tidal bridge, and it formed from the gravitational interactions between these two galaxies.

SN 2005cf has been classified as a Type Ia supernova. Generally, supernovae are classified as one of two types based on whether hydrogen is detected in the supernovae's spectra.

When most people think of a supernova, they probably think of a thing that started off as a really big blue star that initially fused hydrogen into helium in its core, but when the core filled up with helium, it fused the helium into carbon and oxygen and later fused that to form heavier elements until the star's core filled up with iron. Iron cannot be fused to produce energy, so at this point, the star initially collapses, and then a rebound shockwave causes the star to explode. These types of supernovae are Type II supernovae. These are the supernovae where hydrogen is detected because the explosion passes through an outer layer of hydrogen. SN 2005cf is not at all like this.

Instead, most Type Ia supernovae like SN 2005cf are formed in binary systems which contain one white dwarf closely orbiting a larger star such as a Sun-like star or a red giant. The two stars in one of these systems are initially so close together that the white dwarf can gravitationally strip gas from the outer layers of the other star. (I feel like I've been talking about this a lot recently.) Anyway, as a reminder, a white dwarf is the inert core of carbon and oxygen left over after when a Sun-like star dies. The carbon and oxygen normally cannot undergo fusion to form heavier elements because it does not have enough mass. However, if the white dwarf strips enough gas from its companion to get over a mass of about 1.4 times the mass of the Sun, the carbon will be able to undergo very rapid fusion, which will cause the white dwarf to completely explode, leaving behind nothing but its companion star. Generally, no hydrogen is detected in this type of supernova because it involves very little hydrogen. This is a Type Ia supernova, and this is the type of event that produced SN 2005cf.

However, compared to other Type Ia supernovae, SN 2005cf is especially noteworthy for two reasons. First, it occurred in an ideal location to study it. It formed in a structure between two galaxies that is relatively devoid of interstellar dust that could have obscured the light from the supernova, and it appears in the Earth's sky in a location where the interstellar dust from our galaxy is relatively thin and therefore does not affect the supernova's light. Additionally, the supernova, which is at a distance of 91 million light years (27.9 Mpc) [3], is relatively close in terms of supernovae. However, aside from the location of the supernova, astronomers were also sufficiently organized and sufficiently successful at writing science proposals to get a huge amount of observational data for SN 2005cf in the ultraviolet, visible, and infrared parts of the electromagnetic spectrum over a period of three months [4]. This gave everyone detailed information on the exact spectrum of SN 2005cf and on how the supernova faded over time.

This data has been critically important for measuring distances to other Type Ia supernovae. Astronomers have been using Type Ia supernovae to measure distances to other galaxies, and this is possible because all Type Ia supernovae are very similar objects. They all occur when a white dwarf in a binary star system goes over the same mass limit of about 1.4 times the mass of the Sun and explodes. This means that Type Ia supernovae all produce very similar amounts of light, although people have found that the amount of light that the Type Ia supernovae produce is linked to how quickly they fade, but the amount of light produced is still very predictable. Therefore, by measuring the brightness of a Type Ia supernova as seen from Earth and using this information on how much total light the supernova should produce, astronomers can calculate the distances to the supernovae. Because SN 2005cf was so close and so very well studied, astronomers were able to create detailed templates of the spectrum of the supernova that could be used in comparison to other Type Ia supernovae to improve people's abilities to measure the distances to those other objects [4].

This has been extremely important for studying the expansion of the universe. A few years before SN 2005cf appeared, astronomers had been using observations of other Type Ia supernovae to show not only that more distant galaxies were moving away from the Milky Way than closer ones, which we would expect from the standard version of the Big Bang theory, but also that the expansion appeared to be accelerating, which we would not expect from the standard version of the Big Bang theory. This acceleration led to the inference of the presence of a weird force associated with voids in space called dark energy, and it eventually led to a Nobel prize for three of the countless number of people who worked on this research. Anyhow, the data from SN 2005cf have been very important for improving people's abilities to make the distance measurements needed to infer the presence of dark energy, and while the supernova faded away many years ago, its data will live on in astronomy for years to come.


[1] Pugh, H. and Li, W., Supernovae 2005ce and 2005cf, 2005, Central Bureau Electronic Telegrams, 158, 1

[2] Pastorello, A. et al., ESC observations of SN 2005cf - I. Photometric evolution of a normal Type Ia supernova, 2007, Monthly Notices of the Royal Astronomical Society, 376, 1301

[3] Weyant, Anja et al., SweetSpot: Near-infrared Observations of 13 Type Ia Supernovae from a New NOAO Survey Probing the Nearby Smooth Hubble Flow, 2014, Astrophysical Journal, 784, 105

[4] Wang, X. et al., The Golden Standard Type Ia Supernova 2005cf: Observations from the Ultraviolet to the Near-Infrared Wavebands, 2009, Astrophysical Journal, 697, 380


Podcast and Website: George J. Bendo

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