Caltech Astronomy Graduate School Application FAQ

Are you considering graduate study in astronomy at Caltech? Here are answers to some of the basic questions about the department. If you have a question not answered here, please contact the chair of the Admissions Committee.

What is the history of Caltech Astronomy?

Caltech astronomy's beginnings are rooted in George Ellery Hale and the founding of Caltech itself. Fritz Zwicky and Jesse Greenstein arrived in optical astronomy with the creation of Palomar Observatory, and the building of its famed 200" telescope as well as an 18" telescope that would later motivate the 48" sky survey telescope. Soon thereafter came John Bolton in radio astronomy, who established the Owens Valley Radio Observatory. Over the intervening 3/4 century, Caltech has been one of the world's premier astronomical research institutions, and Caltech astronomers have lead numerous fundamental discoveries including those enabled by the strong relationship in the early days with the Carnegie Institution (e.g. Hubble, Baade, and Sandage). The fields of infrared astronomy, radio VLBI, and gravitational-wave astrophysics were born and developed at Caltech. And there are no signs of slowing down.

Why should I apply to Caltech Astronomy?

Currently, Caltech operates what is arguably the world's largest and finest collection of astronomical facilities, covering broad range of wavelengths. Caltech graduate students have every opportunity to participate in both observation and instrumentation efforts at these facilities. Complementing the strong observational/instrumentation program is a strong program in astrophysics theory.

Caltech values its unique status as a "small school with big assets." We prefer to keep our graduate and undergraduate programs somewhat smaller than many of our competitors because this allows us to focus on our mission of education, research training and mentoring, including giving exceptional opportunities with our world-leading observatories and facilities to each and every one of our students. The primary mission of a Caltech graduate education is to provide you with a deep educational background in astronomy and astrophysics, to train and support your doing world-class research as a Ph.D. scholar, and to prepare you for a career beyond an M.S. or PhD. degree. We are consistently ranked one of the best graduate and Ph.D. programs in the world, and our department has produced a tremendous number of leaders and pioneers in their fields. We believe that no other institution can match our combination of research opportunities, access to facilities and data, mentoring focus, and educational depth, as well as results.

What unique resources and facilities will be available to me as a Caltech student?

The two Caltech/UC 10-meter Keck telescopes on Mauna Kea in Hawaii are the world's most capable optical telescopes. They have a wide complement of instrumentation for infrared and optical imaging and spectroscopy, including for adaptive optics. Many graduate theses are based on Keck data, and students frequently travel to Hawaii with their advisors to conduct or to assist with observing runs. Many other students have based theses on the building and commissioning of new instruments, a constant source of activity in our department.

Telescopes at Palomar Observatory are used for both optical and infrared astronomy. They include the Hale 200-inch telescope, a 60-inch telescope, and the 48-inch Oschin Schmidt telescope, The 200-inch telescope is famous for its role in cosmology and in the discovery of quasars and radio galaxies. Recently-developed instruments for the 200-inch telescope include optical and infrared array cameras for imaging and spectroscopy, energy-resolving detectors, an ultra-fast imaging camera, and a state of the art adaptive optics system. The 60-inch is mainly used to follow up interesting targets detected at the 48-inch, and it is also available for independent student projects. The Oschin 48-inch Schmidt telescope, which conducted the celebrated Palomar Sky Survey in the 1950s and again in the 1980s, is now breaking new ground in "time-domain" astronomy as the Zwicky Transient Facility.

Caltech faculty and students have built balloon-, rocket-, and satellite-based cosmic-ray detectors and gamma-ray detectors with unique high-resolution imaging capabilities, and student theses can leverage data from all of these. Some of these projects include BOOMERAnG, BOLOCAM ACBAR, and SPHEREX, ACE, NuSTAR, GALEX, and FIREBALL, spanning wavelengths from microwaves through ultraviolet, X-rays, and gamma-rays. In the non-electromagnetic domain, Caltech and MIT have built the world's first gravitational wave observatory, LIGO, a pair of laser interferometers with 4-km arms. In 2015, these interferometers first detected gravitational waves, and in 2017, they detected the first confirmed neutron star merger. Caltech observatories at all wavelengths work in close concert with the instrument, experiment, data, and theory groups of LIGO to understand these events across all parts of the electromagnetic spectrum before and after gravitational waves are observed.

Caltech graduate students in astronomy use all of the above instruments in their research, as well as national facilities such as the VLA radio telescopes, telescopes at the Cerro Tololo Inter-American Observatory or Kitt Peak National Observatory, and various NASA facilities, including the Hubble Space Telescope, CHANDRA, Spitzer, ALMA, and the GBT.

Close proximity to all this exciting observational activity inspires a vibrant Theoretical Astrophysics group. This comprises 5 faculty (in addition to several theorists in our Planetary Sciences group and Particle Cosmology groups who can advise astronomy students), plus about 10 postdoctoral fellows and a dozen graduate students. Interests include the physics of neutron stars, black holes, and white dwarfs; active galactic nuclei and quasars; the formation of planets, stars and galaxies; cosmology and the origins of large-scale structure; developing new tests of general relativity and determining the nature of dark matter and dark energy; the structure of stars and dynamics of planets; astrophysical fluid dynamics, plasma physics, and the theory of interstellar turbulence. Caltech's computational facilities are no less impressive. Our theory group owns and operates a massively-parallel (>2000 CPU) supercomputer "in house" used primarily for graduate student and postdoc research. Caltech has recently constructed, and is continuously expanding, a new >15,000 CPU+GPU supercomputer for campus-wide use. And Caltech faculty have several large (>100 million CPU-hour) allocations on multiple national (NASA, NSF, DOE) supercomputer facilities ranked among the fastest supercomputers on Earth. In fact, in recent years, Caltech has been awarded more time on these national facilities than any other astrophysics institution in the United States. These resources are used for a wide range of projects ranging from numerical simulations of theoretical phenomena, development of new algorithms, "big data" processing and analysis, reduction of massive interferometric datasets, modeling gravitational waveforms, and more. Caltech's Center for Data-Driven Discovery, a part of a joint initiative in data science and technology with JPL, also pursues a number of projects in "big data" astronomy.

Unique educational resources also exist for interested graduate students to co-mentor undergraduate and high-school students through our Summer Undergraduate Research Fellows program(s) and Center for Teaching, Learning, and Outreach. Some of our students participated in these programs before coming here as graduate students, and it helped to set them on a path of scientific research and give them unique opportunities.

How do I finance my graduate education (and application)?

Our students are not expected to finance their own graduate education: they are supported by Caltech fellowships, by the research funds of Caltech faculty, and by teaching assistantships (whose duties are generally limited to office hours and grading for a small course, as part of your training to be an educator -- teaching and tutoring larger courses can easily be arranged for those interested). Admitted students are guaranteed financial support and are not required to work for any specific sponsoring faculty member. Caltech is a private university. There is therefore no difference in the financial support of California natives, U.S. citizens, and non-citizens. We do encourage our applicants to seek outside fellowships -- e.g. NSF, NASA, Hertz, ATT fellowships for US students, government or other fellowships for foreign students, Zonta fellowships for qualifying women, etc. These outside fellowships recognize your impressive achievements, confer status and can help when applying to future jobs, and often allow you to bring in additional bonus salary and/or research funding for yourself.

If for any reason you fear that financial hardship would prevent you from applying or coming to Caltech, please contact the chair of the Admissions Committee. Application fee waivers can be requested by completing the online fee waiver form within the graduate application (submit it through the online admissions system). For the waiver request, financial hardship can be demonstrated by providing a statement of financial need from a current University or college, or by an explanation of other circumstances where the application fee would provide a financial burden.

Should I apply in Astronomy, Physics, or Planetary Science?

Students interested primarily in planetary astronomy or geophysics should apply for admission to the Planetary Science or Geophysics option. Students interested in cosmochemistry may apply for admission either to the Astronomy option, or to the Geochemistry option, depending on other interests. Students interested in space physics (e.g. cosmic rays), or the particle-physics/cosmology interface, or gravitational-wave detectors (e.g. LIGO instrumentation, as opposed to electromagnetic counterparts of gravitational wave sources) should generally apply for admission in Physics. Students interested in theoretical astrophysics or in any form of observational astronomy may apply either in Astronomy or in Physics.

Regardless of whether you apply in Astronomy, Physics, or Planetary Science, students in any of these options can (and often do) work with faculty across options. The primary difference between the options is in the graduate curriculum. The Astronomy courses give a broad exposure to astrophysics, with emphasis on physical processes rather than purely observational or phenomenological astronomy. The physics courses sample a range of topics across physics: particle/high-energy physics, quantum information, gravity & cosmology, and interdisciplinary fields (e.g. biophysics). Students in either option are encouraged to take courses in the other. Historically, a slight majority of students in theoretical astrophysics, infrared astronomy, high-energy astrophysics, and gravitational-wave detector development have come from the Physics option. A larger majority of those in radio, millimeter wave, optical, and solar astronomy have come from the Astronomy option. Most students in exoplanet astronomy have come from the Astronomy option, but a number have gone through Planetary Science. Applicants should not pay much attention to these historical trends, however. Your choice of option should be based primarily on how certain you are that you want to be an astronomer/astrophysicist, the strength of your previous preparation in physics, and where you feel you may need more background preparation.

What should I have studied before coming to Caltech?

Applicants for graduate study in the astronomy option should be committed to a career in astronomy or astrophysics. While our graduates do go on in some cases to pursue many different career options, long-term association with astronomy is what our program is primarily intended to provide the background for. Most incoming students are strong in undergraduate-level physics and previous exposure to astronomy is helpful, but certainly not required for admission. We also encourage applications from students with weaker preparation in physics, but complementary backgrounds: recent graduate students in astronomy have included a chemistry major, a math major, a computer science major, and a number of students who had worked in industry for several years before coming to Caltech. If you have had to make exceptional efforts or overcome exceptional circumstances in your study, for any reason, please let us know your story. For example, a number of our students went to colleges which did not offer astronomy or upper-level physics courses, and took it upon themselves to find other Universities where they could sit in on classes.

For those who are not native speakers of English: proficiency in English is essential. An exam in both written and spoken English must be passed before you can receive a teaching assistantship. You will not enjoy your courses or research if you cannot understand the lectures and seminars!

What are the specific application requirements? What factors are important in the evaluation of my application?

Everything you send is important! We read it all. One or two outstanding items can outweigh several poor ones in our decisions. There is no "minimum" or "threshold" value for any quantity (grades, scores, research experience) on our application. We look at everything, including involvement in research, any publications or conference presentations, letters of recommendation (these are required), the courses you have taken and your grades, any awards or honors you have received, previous non-research work experience, GRE scores, and your essay/statement of purpose (including any exceptional circumstances you have had to overcome). Every application will be considered in full by the admissions committee.

A note on GRE scores. We recognize the limitations of standardized tests, but we also recognize that test scores can help some otherwise disadvantaged students stand out. For example, we have admitted students who lacked opportunities to engage in research, because they had high test scores and good recommendations. We have admitted students who felt their grades did not reflect the strength of their physics background owing to extenuating circumstances, or who were worried that their grades might be difficult to interpret because they came from a non-traditional undergraduate institution, based on good test scores. We have also admitted many students who had poor test scores because they conducted stand-out research, or had strong letters, or whose grades clearly demonstrated their strong physics background. We read every application holistically, and we do not impose any cut-off in any of the GRE scores. We also understand that requiring the Physics GRE imposes an unnecessary barrier to applying for some students. Therefore, applicants are not required to submit a Physics GRE score. But if you choose not to submit a Physics GRE score, we ask that you please include a short explanatory note (less than 50 words) in your application -- this will not be used to evaluate you, but is for our data-gathering purposes to better craft our admissions policies going forward.

What does the typical progression through the program look like?

Students typically spend 5 years in our program before defending the PhD and graduating. For most of that time, the primary focus is on research. However, during the first year, a large fraction of the time will be spent on education, primarily coursework. During the second year, some portion of time may be spent finishing up coursework and on student training as educators. By the middle or end of the second year, the primary focus should transition to full-time research, with supplementary education activities such as attendance at seminars and other activities geared to broadening your scientific horizons.


At Caltech, students are encouraged to begin research as soon as they arrive on campus. Each graduate student chooses a research advisor (who can be a teaching or research faculty member from Astronomy or Physics or Planetary Sciences), although for the first two years, your official academic advisor is the Astronomy Option Representative (to ensure you have someone else to talk to and ask questions). Students usually begin by choosing a short (6 month to 1 year) research project, in order to explore new scientific areas and get a feel for different groups' and advisors' ways of doing research, without committing to a full thesis. Some students work very closely with their advisors, others are highly independent, and different groups and advisors have distinct styles -- sometimes finding a "good fit" in mentoring style is more important than specific science topics. As a result some students try several projects before committing to a thesis, while in many cases a first-year project turns into a thesis. Some students choose inter-disciplinary projects and have multiple advisors (e.g. an observer and a theorist, or an astronomer and a geologist). Many of these early projects result in published papers.

In all cases, by the end of the second academic year, graduate students should have an identified faculty advisor who has agreed to supervise them in research through and beyond their candidacy. At some point during their third year, students schedule their "candidacy" presentation. This is when our students choose their thesis committees and present the thesis plan to a faculty committee in an official form for the first time. The goal is to get input from a broad spectrum of faculty, not just your advisor, on how you can maximize your time and impact of your research. Your thesis committee will check in on you from time to time and be a resource available to you, beyond your advisor, anytime you need. The PhD project typically takes 2 years to complete from the time of the candidacy exam.


In addition to research, students in the first year are expected to take a course sequence that provides a complete introduction to advanced astronomy and astrophysics. Although it is true that much of your graduate education comes in the form of research, talks and seminars, and conferences, our former students consistently tell us that their courses provided the essential background to be able to actually follow along and understand these talks and see the connections to their own science (especially in the areas that are not their own specific research topics). Caltech has a reputation for consistently graduating students who are exceptionally well-prepared in their background and broad astrophysics education, which is absolutely critical in a rapidly-evolving field like astronomy (where most people will work on many different topics over the course of a career)!

Importantly, Caltech operates on the academic year quarter system with three 10-week terms per quarter rather than two 15-week terms per semester, so our required curriculum of 10 courses is equivalent, in actual course-hours, to ~6 classes at most of our peer (semester-based) institutions. Most students are able to complete all of their classes in the first year, although sometimes 1-2 high-level elective courses are deferred to second year.

After the first year, our students take a general qualifying exam consisting of a short presentation on their first-year research followed by a brief oral Q&A with questions drawn specifically from their required core courses.

All of our students participate in the graduate student seminars, where they take turns presenting short talks on recent papers in a friendly audience of other students and faculty mentors. This is one of the most popular components of our graduate program and provides an excellent opportunity for honing your speaking and presentation skills.

Teaching & Outreach:

Formally, all Caltech students are required to serve at least one term (one quarter) as a teaching assistant, over the course of their graduate career. In the Astronomy option, most students TA for two or three quarters, generally all during the second year. Although research is expected to be your primary work focus as a graduate student, we are also training educators and believe this provides critical experience. Furthermore, our TA requirement is unlike many institutions where teaching assistantships constitute most of the financial support for students. While most Caltech classes in Astronomy are small (4-10 students), opportunities for teaching sections of large lecture classes are available for interested students.

Many of our graduate students also choose to mentor undergraduate or high-school students through our Summer Undergraduate Research Fellows program(s) and Center for Teaching, Learning, and Outreach, although this is not required. These programs provide resources for supporting students, as well as pedagogical training.

We also have an extensive and vibrant community and public volunteer outreach program. Again this is not required, but most of our students choose to participate in or organize (at varying levels) the diverse array of outreach activities. Opportunities also exist to develop new outreach programs. We estimated over ten thousand members of the public attended Caltech astronomy outreach events last year!

What will my interactions with faculty be like at Caltech?

Approximately 30 members of the Caltech faculty are involved in research in astronomy and astrophysics, as are about 30 postdoctoral fellows and research associates. There are approximately 30 graduate students in astronomy, plus another 25 physics students also doing research in astronomy and astrophysics.

You will interact with Caltech professorial faculty in the classroom and through your research, as well as during talks and seminars, and informally around the Cahill building. Most professors have open doors and are happy to be interrupted to talk about research. Most graduate students begin working on a research project with one or more faculty members during or even before their first quarter at Caltech.

You will also have ample opportunity to meet faculty and visitors at various casual events, such as

Graduate students in our Division (physics, math, and astronomy) are served by a Graduate Program Director. She is the resource for any concerns that students might have about the program. The Astronomy Option Representative is another resource for academic matters. The Option Rep will meet with you regularly to discuss your progress toward academic milestones, culminating in graduation. Finally, the graduate students nominate a Graduate Representative who represents the department at campus-wide meetings, such as the bi-annual Graduate Student-Faculty Colloquium.

How does cost-of-living compare to a grad student stipend?

The typical after-tax paycheck is about $2500 per month. In 2018, the second-year Astronomy graduate students assembled some data on their income and expenses. They identified two limiting case studies: the Frugal Student and the Spendthrift. Every month, the Frugal Student spends $300 on food, $655 on rent and utilities in a shared apartment, and about $1000 on other expenses. He still puts $550 into a savings account or other investments every month! The Spendthrift spends $520 on food, $1195 on rent and utilities (living alone), and about $600 on other expenses. He still saves $200 per month.

What about the quality of life in and around Caltech?

The Caltech campus is located in the pleasant city of Pasadena, 10 miles northeast from downtown Los Angeles, at the foot of the San Gabriel Mountains. Caltech has a beautiful tree-lined campus dominated by Spanish-style . There are approximately 1000 PhDs (300 faculty, 700 postdoctoral fellows), 1200 graduate students, and 950 undergraduates. The undergraduates major primarily in technical fields, but receive a broad education in the humanities as well, while graduate studies concentrate exclusively on science and engineering. Although a small school, Caltech offers a fair number of campus activities. A large number of clubs and special-interest groups are supported by the school. Grad students participate in a variety of athletic activities, ranging from competitive intercollegiate club teams to highly non-competitive summer softball. Professional theater and music groups visit campus regularly, and some 15% of the total student body performs regularly in the Caltech orchestra, chamber music program, Glee Club (choir), jazz band, wind ensemble, or musical theater program. Astronomy students are active in science education, outreach, and advocacy. For example, a current astronomy graduate student founded the club for Women in Physics, Math, and Astronomy (WiPMA) at Caltech. We also host public lectures, "Astronomy on Tap" at a bar in Old Town Pasadena, and Science Train, where we chat up strangers on the L.A. area light rail.

What else is interesting in Pasadena and L.A.?

In addition to Caltech, Pasadena is home to the nation's top design school, the Art Center College of Design, Pasadena City College, and the Fuller Theological Seminary. Within a 2 mile circle around Caltech are located the Norton Simon Museum and the Huntington Museum, Library and Garden, together representing one of the finest collections of European art and manuscripts in the US. Also in that circle are the USC Pacific Asia Museum, the unusual Kidspace interactive children's museum, and the Pasadena Playhouse (the State Theater of California, training ground for many famous film and theater actors and origin of many shows that ultimately become Broadway hits), the trendy nightlife of Old Town Pasadena, and the Pasadena Civic Auditorium, home to the Pasadena Symphony, the Emmy Awards ceremony, and many other events. Within a 4 mile circle around Caltech are located Pasadena's celebrated Rose Bowl stadium, the Southwest Museum, Eaton Canyon and Arroyo Seco recreation areas, the Los Angeles County Arboretum, and the Santa Anita horse racing track. Pasadena is home to the distinguished Pasadena Symphony, the Coleman Chamber Music Association (whose concerts are held at Caltech), and numerous rock bands, bars, nightclubs, and comedy clubs.

Slightly further afield, in the greater Los Angeles, one can find the LA Civic Center, Walt Disney Concert Hall (opera, theater, LA Symphony), the Hollywood Bowl, Chinatown (old and new), and the sports stadia of Los Angeles e.g. Dodger Stadium and Staples Center. Access to central LA is now possible by the Metro Gold Line light rail. In a 15 mile radius are several hundred live-performance theaters, the movie studios of Hollywood, and the television studios of Burbank. Caltech is on occasion a popular filming location. Disneyland, Universal Studios, Magic Mountain, and Knotts' Berry Farm are an hour's drive away.

Hikers and mountain bikers enjoy the San Gabriel mountains and Angeles National Forest, just north of Pasadena. The quickest access to hiking trails is only a half hour away, with many more remote trail options ranging up to 1.5 hours away. During winter months, a 2 hour drive will take you to downhill skiing at 9,000-ft elevation and a 5 hour drive to the premiere skiing available up to 11,000 feet at Mammoth Mountain in the Sierra Nevada. In the other direction, less than an hour's drive across Los Angeles (at certain times of day) will take you to the closest beaches of the Pacific Ocean. Joshua Tree National Park is less than 2.5 hours to the east.

What about other astronomy and technology?

Just up the road in La Canada is JPL, the Jet Propulsion Lab, which grew out of early rocketry experiments by Caltech scientists who were eventually banned to a wilderness location beyond the orange groves of Pasadena. Today, Caltech manages JPL for NASA and some of our previous and recent students have worked "on lab" with collaborative projects between JPL scientists and engineers, and Caltech faculty.

Other institutions in southern california with active astronomy communities include: IPAC (on the Caltech campus), The Observatories of the Carnegie Institution for Science (also in Pasadena), Pomona College, Cal Poly Pomona, USC, UC Los Angeles, UC Santa Barbara, UC Riverside, UC Irvine, UC San Diego.

Beyond astronomy, Pasadena is home to many technology companies. Prominent names include: the Parsons Corporation (the nation's top engineering design firm and one of the largest construction firms), Jacobs Engineering group (NYSE:JEC), Green Dot Corporation (NYSE:GDOT), one of the largest financial companies serving lower income communities with a suite of high tech products, the cybersecurity company Guidance (NASDAQ: GUID), and Western Asset Management company, a large fixed-income manager and a division of Legg Mason. In addition, Pasadena features a vibrant startup scene with incubators (e.g. IdeaLab), robotics companies, A-I companies and digital media companies, among others. The other major technology hub in southern California is colloquially noted as "Silicon Beach." The location, just north of LAX, hosts corporate offices for many major tech firms.