• Hi!
    I'm Sandor

    Astrophysicist and space data scientist at ESA.

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About Me

Who Am I?

Hi I'm Sandor Kruk, Research Fellow at the European Space Agency in the Netherlands. I have wide research interests regarding the analysis of Big Data astronomical surveys, including the detection and classification of galaxies, asteroids and gravitational lenses with citizen science and artificial intelligence. My scientific interests lie in the formation and evolution of galaxies across the cosmic history, galaxy morphology, galaxy dynamics and star formation.

I am of Hungarian ethnicity, but was borned and grew up in the north of Romania, in the beautiful region of Maramures. I discovered astronomy in high school and have been in love with it ever since. Besides research, I have a strong passion for science education and public outreach - I have lead various initiatives to increase access, diversity and inclusion in science, both in my home country and abroad. You can find out more about them below.


Space Data Science

Citizen Science

Education and Public Outreach

What I do?

Ongoing projects

Hubble Asteroid Hunter

Project Principal Investigator. Citizen science project to identify asteroids observed serendipituously in images of the Hubble Space Telescope. Data mining of archives and identification of rare objects. Collaborating with scientists at the the ESA Science Data Centre & Google.

ESA Euclid Mission

Leading a project on the detailed visual morphology of galaxies which will be observed by the ESA Euclid mission, to be launched in 2022. Member of the Euclid Consortium Galaxy & AGN Evolution, Solar System Objects Science Working Groups and the ESA-ESTEC Euclid team.

Galaxy Zoo

Core member of the Galaxy Zoo and Zooniverse teams. Building citizen science projects in astronomy and space science. Communicating and engaging with citizen scientists in research projects related to galaxy morphology, asteroids and gravitational lensing.



Research Fellow at the European Space Agency, Netherlands 2018-2021

  • Independent research fellowship in space science at ESA-ESTEC (Netherlands). Mentor: Dr René Laureijs (Euclid project scientist)
  • Working with Hubble Space Telescope data and planning for the future ESA Euclid mission
  • Leading citizen science and machine learning projects, in collaboration with scientists at the ESA Science Data Centre (ESDC) in Madrid
  • Functional work includes supporting the ESA Voyage 2050 long-term planning of science missions
  • Bachelor and Master in Astrophysics at University College London, UK 2010-2014

  • Master Thesis: Cosmic ray acceleration in Fermi Bubbles. Advisor: Prof Kinwah Wu (MSSL)
  • Graduated top of class. Prizes for the Best Performance every year in the MSci Astrophysics degree at UCL
  • Commended on Dean's List for being in top 5% undergraduates of the Mathematical & Physical Sciences Faculty at UCL
  • High School Education at "Vasile Lucaciu" National College in Baia Mare, Romania 2006-2010

  • Specialised in Mathematics, Informatics and English
  • Graduated top of class. Romanian Baccalaureate exam (Romanian, Mathematics and Physics) GPA: 9.71/10
  • Member of the Romanian team of five students and medalist in the international astronomy and astrophysics olympiads 2008-2010: Gold, Silver, Bronze Medals, and Honourable Mention

  • My published work


    Links to my ADS Publications and arXiv Preprints
    Google Scholar h-index: 11 and ORCID iD. Selected first author publications below.

    Vertically thickened bars, observed in the form of boxy/peanut (B/P) bulges, are found in the majority of massive barred disc galaxies in the local Universe, including our own. B/P bulges indicate that their host bars have suffered violent bending instabilities driven by anisotropic velocity distributions. We investigate for the first time how the frequency of B/P bulges in barred galaxies evolves from z = 1 to z ≈ 0, using a large sample of non-edge-on galaxies with masses logM⋆ > 10 logM☉, selected from the HST COSMOS survey. We find the observed fraction increases from 0^+3.6_0.0 per cent at z = 1 to 37.8^+5.4_-5.1 per cent at z = 0.2. We account for problems identifying B/P bulges in galaxies with low inclinations and unfavourable bar orientations, and due to redshift-dependent observational biases with the help of a sample from the Sloan Digital Sky Survey, matched in resolution, rest-frame band, signal-to-noise ratio and stellar mass and analysed in the same fashion. From this, we estimate that the true fraction of barred galaxies with B/P bulges increases from ∼10 per cent at z ≈ 1 to ∼ 70 per cent at z = 0. In agreement with previous results for nearby galaxies, we find a strong dependence of the presence of a B/P bulge on galaxy stellar mass. This trend is observed in both local and high-redshift galaxies, indicating that it is an important indicator of vertical instabilities across a large fraction of the age of the Universe. We propose that galaxy formation processes regulate the thickness of galaxy discs, which in turn affect which galaxies experience violent bending instabilities of the bar.

    Link to paper

    We present the results of two-component (disc+bar) and three-component (disc+bar+bulge) multiwavelength 2D photometric decompositions of barred galaxies in five Sloan Digital Sky Survey (SDSS) bands (ugriz). This sample of ∼3500 nearby (z < 0.06) galaxies with strong bars selected from the Galaxy Zoo citizen science project is the largest sample of barred galaxies to be studied using photometric decompositions that include a bar component. With detailed structural analysis, we obtain physical quantities such as the bar- and bulge-to-total luminosity ratios, effective radii, Sérsic indices and colours of the individual components. We observe a clear difference in the colours of the components, the discs being bluer than the bars and bulges. An overwhelming fraction of bulge components have Sérsic indices consistent with being pseudo-bulges. By comparing the barred galaxies with a mass-matched and volume-limited sample of unbarred galaxies, we examine the connection between the presence of a large-scale galactic bar and the properties of discs and bulges. We find that the discs of unbarred galaxies are significantly bluer compared to the discs of barred galaxies, while there is no significant difference in the colours of the bulges. We find possible evidence of secular evolution via bars that leads to the build-up of pseudo-bulges and to the quenching of star formation in the discs. We identify a subsample of unbarred galaxies with an inner lens/oval and find that their properties are similar to barred galaxies, consistent with an evolutionary scenario in which bars dissolve into lenses. This scenario deserves further investigation through both theoretical and observational work.

    Link to paper

    We use multiwavelength Sloan Digital Sky Survey (SDSS) images and Galaxy Zoo morphologies to identify a sample of ∼270 late-type galaxies with an off-centre bar. We measure offsets in the range 0.2-2.5 kpc between the photometric centres of the stellar disc and stellar bar. The measured offsets correlate with global asymmetries of the galaxies, with those with largest offsets showing higher lopsidedness. These findings are in good agreement with predictions from simulations of dwarf-dwarf tidal interactions producing off-centre bars. We find that the majority of galaxies with off-centre bars are of Magellanic type, with a median mass of 10^9.6 M☉, and 91 per cent of them having M⋆ < 3 × 10^10 M☉, the characteristic mass at which galaxies start having higher central concentrations attributed to the presence of bulges. We conduct a search for companions to test the hypothesis of tidal interactions, but find that a similar fraction of galaxies with offset bars have companions within 100 kpc as galaxies with centred bars. Although this may be due to the incompleteness of the SDSS spectroscopic survey at the faint end, alternative scenarios that give rise to offset bars such as interactions with dark companions or the effect of lopsided halo potentials should be considered. Future observations are needed to confirm possible low-mass companion candidates and to determine the shape of the dark matter halo, in order to find the explanation for the off-centre bars in these galaxies.

    Link to paper
    Where my work appeared


    Foreground asteroid passing the Crab Nebula

    ESA Science Image of the Week

    One galaxy, two asteroids

    ESA Science Image of the Week

    Hubble’s celestial peanut

    ESA Science Image of the Week

    My projects

    Education Projects

    Coming from a country with a small representation in the international astronomy and science research community, I have a strong passion for helping others to discover their passion for science and enable them to succeed in their careers. Over the last five years, I have set up three initiatives to support young people in their scientific journey. Through these initiatives, thousands of students engage with science every year and have access to world-class researchers from all over the world as mentors. You can read about these projects below.

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    Non-formal science education | Science Festival |

    Romanian Science Festival

    Initiated and coordinated the first national science festival in Romania. Organised in 3 cities in 2019, with over 700 volunteers, 50 mentors and 20,000 participants. Moved online in 2020, with regular science webinars and scientists joining online classes.

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    Education | Summer School |

    Oxford for Romania

    Co-founded a yearly multi-disciplinary summer school for 25 bright Romanian students from disadvantaged backgrounds organised in Oxford colleges. Empower young people to discover their talents, encounter new career perspectives and participate in the society.

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    Astronomy Education | Olympiad |

    British Astronomy and Astrophysics Olympiad

    Founded the first national Astronomy and Astrophysics olympiad in the UK, with 1000+ participants every year. Gateway to the International Olympiads in Astronomy and Astrophysics.

    My projects

    Outreach and Public Engagement

    I enjoy communicating science and have given many public talks to a diverse range of audiences, both in person and online. If you would like me to give a talk please get in touch using the contact below. I organise workshops popularising astronomy and space at science festivals and open days. I coordinate a program connecting researchers from abroad with classrooms in Romania, giving young people the opportunity to meet real scientists.

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    Space Talks | Webinars |

    Outreach Talks

    Latest pulic talks (2020) include: ESA Science Missions - Exploring the unknown, EUROAVIA, Bucharest; 30 years of the Hubble Space Telescope, ESERO "Bringing Space to your Homes" (online); Three (astronomical) ways the world will end, Online webinar.

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    Non-formal education | Experiments |

    Science Workshops

    I am regularly invited to science festivals and open days to organise science workshops and present experiments to the public. Latest events include: Ireland Maths Festival, Oxford Science Festival, ESA Open Days.

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    Education | Role Models |

    Invite a mentor to your online classroom

    To support the education during the pandemic, I created a platform where scientists can connect with teachers and participate in online classrooms. In April-June 2020, 25 researchers participated in 55 classes, reaching 1000+ young people in Romania.

    Get in Touch


    European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands